fix(Raster): update default dtypes to include unsigned integers

[jlarsen-usgs]

Close #1144

[codecov]

Codecov Report

Merging #1147 (b534cd3) into develop (345e0fb) will increase coverage by 1.482%.
The diff coverage is 100.000%.

❗ Current head b534cd3 differs from pull request most recent head 6e73841. Consider uploading reports for the commit 6e73841 to get more accurate results

@@              Coverage Diff              @@
##           develop     #1147       +/-   ##
=============================================
+ Coverage   72.093%   73.576%   +1.482%     
=============================================
  Files          225       225               
  Lines        52002     50878     -1124     
=============================================
- Hits         37490     37434       -56     
+ Misses       14512     13444     -1068     

Impacted Files	Coverage Δ
flopy/utils/rasters.py	56.589% <100.000%> (ø)
flopy/modflow/mfswi2.py	62.461% <0.000%> (-9.847%)	⬇️
flopy/modflow/mfoc.py	91.732% <0.000%> (-2.953%)	⬇️
flopy/modflow/mfbcf.py	87.939% <0.000%> (-2.011%)	⬇️
flopy/utils/datafile.py	72.811% <0.000%> (-1.383%)	⬇️
flopy/utils/mfreadnam.py	76.642% <0.000%> (-0.730%)	⬇️
flopy/utils/flopy_io.py	75.641% <0.000%> (-0.428%)	⬇️
flopy/mf6/modflow/__init__.py	100.000% <0.000%> (+4.838%)	⬆️
flopy/mf6/modflow/mfgnc.py	52.941% <0.000%> (+20.798%)	⬆️
flopy/mf6/modflow/mfmvr.py	52.380% <0.000%> (+20.952%)	⬆️
... and 56 more