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

Patch coverage: 50.00% and project coverage change: -0.04 ⚠️

Comparison is base (8ded81c) 87.25% compared to head (335619d) 87.21%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #461      +/-   ##
==========================================
- Coverage   87.25%   87.21%   -0.04%     
==========================================
  Files          23       23              
  Lines        3820     3824       +4     
==========================================
+ Hits         3333     3335       +2     
- Misses        487      489       +2     

Impacted Files	Coverage Δ
gwcs/coordinate_frames.py	94.93% <50.00%> (-0.44%)	⬇️

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