Add a multi-dimensional PixelFrame

[Cadair]

I often find myself doing cf.CoordinateFrame(3, ["PIXEL", "PIXEL", "PIXEL"], unit=[u.pix, u.pix, u.pix], ...) to generate an input frame to a single stage "pixel to world" type WCS.

Having a shortcut, designed to be an input frame where you aren't going to have a compound frame or anything as input, like this PixelFrame would let you do PixelFrame(3) and get the same result.

I do think that Frame2D feels somewhat like a special case of this new PixelFrame, but not sure if it's worth deprecating Frame2D or not.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 87.66%. Comparing base (8a28efc) to head (2d6ee0d).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
gwcs/coordinate_frames.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #461      +/-   ##
==========================================
+ Coverage   87.58%   87.66%   +0.08%     
==========================================
  Files          22       22              
  Lines        3955     3908      -47     
==========================================
- Hits         3464     3426      -38     
+ Misses        491      482       -9     

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