Prevent creation of WCS with (or insertion of) duplicate frames

[mcara]

In https://github.com/spacetelescope/jwst/pull/2629#discussion_r221419842 @nden said:

all frame names in the pipeline must be unique

In this issue I propose that this be enforced when the WCS object is created or its pipeline modified.

Currently, when using the standard example from the [gWCS docs] modified to have two identical frames, it does not raise an exception: wcsobj = wcs.WCS([('det', det2sky), ('det', det2sky), ('sky', None)]). But the code does crash when attempting to evaluate forward transform:

In [19]: wcsobj(10,10)
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-19-a8018916eccf> in <module>()
----> 1 wcsobj(10,10)

~/.../lib/python3.5/site-packages/gwcs/wcs.py in __call__(self, *args, **kwargs)
    242             kwargs['fill_value'] = np.nan
    243 
--> 244         if self.bounding_box is not None:
    245             # Currently compound models do not attempt to combine individual model
    246             # bounding boxes. Get the forward transform and assign the ounding_box to it

~/.../lib/python3.5/site-packages/gwcs/wcs.py in bounding_box(self)
    456         transform_0 = self.get_transform(frames[0], frames[1])
    457         try:
--> 458             bb = transform_0.bounding_box
    459         except NotImplementedError:
    460             return None

AttributeError: 'NoneType' object has no attribute 'bounding_box'

However, this 'bounding_box' error is not intuitive or directly pointing to the underlying issue.

[stscijgbot]

This ticket is now being tracked at AL-28