feat(gridutil): add function to help create DISV grid

[langevin-usgs]

This PR adds a simple function that constructs vertices, cell2d, and other arguments needed to construct a flopy ModflowGwfdisv object. This function is mostly designed for MODFLOW testing purposes, but it may also be generally useful for constructing a DISV grid. Could be enhanced to remove IDOMAIN=0 cells and to rotate the resulting grid.

    disvkwargs = get_disv_kwargs(
        nlay, nrow, ncol, delr, delc, top, botm, xoff, yoff,
    )
    disv = flopy.mf6.ModflowGwfdisv(gwf, **disvkwargs)

[codecov]

Codecov Report

Merging #1952 (a8148c5) into develop (2fa32da) will decrease coverage by 1.8%.
The diff coverage is 90.6%.

@@            Coverage Diff            @@
##           develop   #1952     +/-   ##
=========================================
- Coverage     72.6%   70.9%   -1.8%     
=========================================
  Files          257     257             
  Lines        57711   57754     +43     
=========================================
- Hits         41927   40968    -959     
- Misses       15784   16786   +1002     

Files Changed	Coverage Δ
flopy/utils/gridutil.py	96.0% <90.6%> (-2.5%)	⬇️

... and 55 files with indirect coverage changes