feat(get-modflow): allow specifying repo owner

[wpbonelli]

• support --owner cli option and corresponding run_main() (and other fn) params
• document owner option in get_modflow.md
• expand tests

This allows installing from releases posted on e.g. personal forks of modflow6. For instance, after this PR, to install the PRT development build:

get-modflow : --owner aprovost-usgs --repo modflow6

[codecov]

Codecov Report

Merging #1910 (57bf4e5) into develop (c25c0b3) will decrease coverage by 0.1%.
The diff coverage is 81.8%.

@@            Coverage Diff            @@
##           develop   #1910     +/-   ##
=========================================
- Coverage     72.5%   72.5%   -0.1%     
=========================================
  Files          257     257             
  Lines        56261   56267      +6     
=========================================
- Hits         40843   40842      -1     
- Misses       15418   15425      +7     

Files Changed	Coverage Δ
flopy/utils/get_modflow.py	63.7% <81.8%> (-0.2%)	⬇️

... and 1 file with indirect coverage changes

[langevin-usgs]

This looks good to me, @w-bonelli