fix bug in hexgrid and documentation

[syhaffert]

I have cleared up the documentation of make_hex_grid and I have switched some internal variable names to use the actual correct geometric properties.

I have run a couple of tests with the TMT and ELT aperture to make sure they still create identical apertures and those tests passed.

[codecov[bot]]

Codecov Report

Merging #168 (283ce67) into master (6aebeb9) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #168   +/-   ##
=======================================
  Coverage   80.70%   80.70%           
=======================================
  Files          95       95           
  Lines        7044     7045    +1     
=======================================
+ Hits         5685     5686    +1     
  Misses       1359     1359           

Impacted Files	Coverage Δ
hcipy/field/util.py	66.84% <100.00%> (+0.17%)	:arrow_up:

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[syhaffert]

@ehpor I think we can actually close this PR because we decided that it was not an implementation error.

[ehpor]

@syhaffert Agreed. I might make a separate PR clarifying the language a bit, but that's a separate PR.