Validate schemas

[kosack]

When writing sub-classes, the parent classes in fits_schema should do some sanity checks like:

• keyword duplication
• conflicts with FITS standard (e.g if you define TELESCOP in your schema, it should not change the type expected by the FITS standard, etc)

This could be done by applying validations using parent classes, or just by explicitly checking all keywords against FITS standards.

[maxnoe]

This could be done by applying validations using parent classes, or just by explicitly checking all keywords against FITS standards.

I think this logic needs to take place in HeaderSchemaMeta.__new__, you have all the different header schemas available there and can check them for consistency. Like this, it's even a definition time error to create an invalid schema

[kosack]

The only issue now is that the way it currently works is that some header keywords are defined in e.g. BinaryTableHeader, and then can be overridden in user-defined schemas, i.e.. In that case, this is not a duplicated keyword, but an overridden one. Have to think a bit about how that should work. Either we don't pre-define all the potential headers, or we just add some metadata to the core ones that are allowed to be overridden, e.g.:

class BinaryTableHeader(Header):
    ...
    EXTNAME = HeaderCard(required=False, type_=str, allow_override=True) # this can be overridden

class MyHeader(Header):
    EXTNAME = HeaderCard(allowed_values="EFF_AREA", ...)  # should not cause a duplicate key error