Add boudning_box to user_guide

docs/user_guide/boundingbox.md

@@ -0,0 +1,124 @@
+# Bounding Box
+
+## Basic Example
+
+### Polytope-mars
+
+A basic example of requesting a trajectory using polytope-mars:
+
+```python
+from polytope_mars.api import PolytopeMars
+
+request = {
+    "class" : "od",
+    "stream" : "enfo",
+    "type" : "pf",
+    "date" : "20240930",
+    "time" : "0000",
+    "expver" : "0079", 
+    "domain" : "g",
+    "param" : "164/167/169",
+    "levtype" : "pl",
+    "number" : "1",
+    "feature" : {
+        "type" : "boundingbox",
+        "points" : [[-1, -1], [1, 1]],
+	},
+    "format" : "covjson",
+}
+
+result = PolytopeMars().extract(request)
+```
+
+This request will return a bounding box with forecast date of `20240930T000000` for the three requested parameters for the points within a bounding box with top left coordinate at latitude -1 and longitude -1, and bottom right point at latitude 1 and longitude 1.
+
+
+
+Notes: 
+* The data has to exist in the data source pointed to in the config.
+* No config is provided via the PolytopeMars interface so a config will be loaded from the default locations. The config can also be passed directly via the interface.
+
+### Earthkit-data
+
+An example of the same request above except requested via Earthkit-data:
+
+```python
+import earthkit.data
+
+request = {
+    "class" : "od",
+    "stream" : "enfo",
+    "type" : "pf",
+    "date" : "20240930",
+    "time" : "0000",
+    "expver" : "0079", 
+    "domain" : "g",
+    "param" : "164/167/169",
+    "levtype" : "sfc",
+    "number" : "1",
+    "feature" : {
+        "type" : "boundingbox",
+        "points" : [[-1, -1], [1, 1]],
+	},
+    "format" : "covjson",
+}
+
+ds = earthkit.data.from_source("polytope", "ecmwf-mars", request, stream=False, address='polytope.ecmwf.int')
+```
+
+The data returned will be the same as the request above. `"polytope"` refers to the underlying service being used to return the data. `"emcwf-mars"` is the dataset we are looking to retrieve from. Setting `stream=False` returns all the requested data to us once it is available. `address` points to the endpoint for the polytope server.
+
+Notes: 
+* The data has to exist in the fdb on the polytope server.
+* No config is required to be passed when using this method, it is generated on the server side.
+* Further details on the `from_source` method can be found here: https://earthkit-data.readthedocs.io/en/latest/guide/sources.html
+
+## Required Fields
+
+For a boundingbox within the `feature` dictionary two fields are required
+
+* `type`
+* `points`
+
+For a bounding box `type` must be `boundingbox`.
+
+`points` must contain two points, the first corresponding to the top left of the requested box, and the second correspongin to the bottom right coordinate. By default they should only contain a latitude and longitude. However as seen below this can be changed with the `axes` key.
+
+
+## Optional Fields
+
+`axes` refers to the axes on which to generate the bounding box. As stated above the minimum default `axes` contains `lat` and `long` meaning if `axes` is not included these values must be provided per point. By default the level is taken from the main body of the request.
+
+However `axes` can also be provided by the user and with a value for level. Such as here:
+
+```python
+"axes" : ["lat", "long", "level"]
+```
+
+In this case the user must provide a `lat`, `long` and `level`. `level` should not be included in the main body of the request in this case. An example can be seen here:
+
+
+```python
+request = {
+    "class" : "od",
+    "stream" : "enfo",
+    "type" : "pf",
+    "date" : "20240930",
+    "time" : "0000",
+    "expver" : "0079", 
+    "domain" : "g",
+    "param" : "164/167/169",
+    "levtype" : "pl",
+    "number" : "1",
+    "feature" : {
+        "type" : "boundingbox",
+        "points" : [[-1, -1, 1000], [1, 1, 500]],
+        "axes" : ["lat", "long", "level"],
+	},
+    "format" : "covjson",
+}
+```
+
+For this request a bouding box with top left corner at lat -1, long -1 and pressure level 1000, and bottom right corner at lat 1, long 1, and pressure level 500.
+
+Without level in the `axes` this will be taken from the main body of the request. In the case of `levtype` = `sfc`, no levelist is required.