DitheredObservation

Depends on panoptes#735

* Rearrange to support mulitple types of `Observation`s.
* Add a `DitheredObservation` (coming from AstroHuntsman/huntsman-pocs).

pocs/scheduler/observation/__init__.py

@@ -0,0 +1 @@
+from pocs.scheduler.observation.base import Observation  # pragma: no flakes

pocs/scheduler/observation/dithered.py

@@ -0,0 +1,81 @@
+from astropy import units as u
+
+from contextlib import suppress
+from pocs.scheduler.field import Field
+from pocs.scheduler.observation import Observation
+
+from pocs.utils import listify
+
+
+class DitheredObservation(Observation):
+
+    """ Observation that dithers to different points.
+
+    Dithered observations will consist of both multiple exposure time as well as multiple
+    `Field` locations, which are used as a simple dithering mechanism.
+
+    Note:
+        For now the new observation must be created like a normal `Observation`,
+        with one `exp_time` and one `field`. Then use direct property assignment
+        for the list of `exp_time` and `field`. New `field`/`exp_time` combos can
+        more conveniently be set with `add_field`
+    """
+
+    def __init__(self, *args, **kwargs):
+        super(DitheredObservation, self).__init__(*args, **kwargs)
+
+        # Set initial list to original values
+        self._exp_time = listify(self.exp_time)
+        self._field = listify(self.field)
+
+        self.extra_config = kwargs
+
+    @property
+    def exp_time(self):
+        exp_time = self._exp_time[self.exposure_index]
+
+        if not isinstance(exp_time, u.Quantity):
+            exp_time *= u.second
+
+        return exp_time
+
+    @exp_time.setter
+    def exp_time(self, values):
+        assert all(t > 0.0 for t in listify(values)), \
+            self.logger.error("Exposure times (exp_time) must be greater than 0")
+
+        self._exp_time = listify(values)
+
+    @property
+    def field(self):
+        return self._field[self.exposure_index]
+
+    @field.setter
+    def field(self, values):
+        assert all(isinstance(f, Field) for f in listify(values)), \
+            self.logger.error("All fields must be a valid Field instance")
+
+        self._field = listify(values)
+
+    @property
+    def exposure_index(self):
+        _exp_index = 0
+        with suppress(AttributeError):
+            _exp_index = self.current_exp_num % len(self._exp_time)
+
+        return _exp_index
+
+    def add_field(self, new_field, new_exp_time):
+        """ Add a new field to observe along with exposure time
+
+        Args:
+            new_field (pocs.scheduler.field.Field): A `Field` object
+            new_exp_time (float): Number of seconds to expose
+
+        """
+        self.logger.debug("Adding new field {} {}".format(new_field, new_exp_time))
+        self._field.append(new_field)
+        self._exp_time.append(new_exp_time)
+
+    def __str__(self):
+        return "DitheredObservation: {}: {}".format(self._field, self._exp_time)