Define and check position angles

[cdeil]

At the moment we have a pa key for spatial model position angle in our schema:

gamma-cat/input/schemas/dataset_source_info.schema.yaml

Line 142 in 786e5d1

pa:

and we have ~ 20 files where this info was added:
https://github.com/gammapy/gamma-cat/search?p=3&q=%27pa%3A%27&type=&utf8=%E2%9C%93

Currently position angles aren't well-defined, they all need a frame (radec or galactic), and they need to be uniform in their definition (e.g. "east or north" or something like that).
The tasks here are to:

• Change the schema to support data entry with multiple frames and orientations so that data entry can be done as it is in the paper, without thinking and room for error. Presumably add an "orientation" string field.
• Change the scripts to process that new "orientation" parameter and convert to a new uniform orientation.
• Check the data entry for all ~ 20 sources we have already against the paper, to make sure it's correct. (in many cases a helpful comment concerning orientation from the paper was already left in the YAML file).

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Until someone implements this, people doing data entry should always leave a one-line comment above the pa: line, stating the orientation as mentioned in the paper.