Allow generating a HLO from intermediate frames

[nden]

This implements the request in #644 (comment). provide an option to generate a. high level objest when evaluating a transform between intermediate frames.
The implementation needs a change in astropy (PR here).

This PR is based on #660.
The requested functionality is shown in test_transform_intermediate_1d test.

[codecov]

Codecov Report

❌ Patch coverage is 93.82716% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.46%. Comparing base (07a7538) to head (2264cf3).
⚠️ Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
gwcs/wcs/_wcs.py	93.10%	4 Missing ⚠️
gwcs/coordinate_frames/_base.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #667      +/-   ##
==========================================
+ Coverage   86.18%   86.46%   +0.28%     
==========================================
  Files          39       39              
  Lines        4111     4146      +35     
==========================================
+ Hits         3543     3585      +42     
+ Misses        568      561       -7     

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