bug(flopy_io): limit fixed point format string to fixed column widths

[jdhughes-usgs]

flopy_io.write_fixed_var could write a fixed point string (f) that
exceeded the specified fixed column width. Added test to revert to
general format (g) with precision equal to column width - 6 to preserve
as much precision as possible. This value allows for a string with a
sign (+/-) and +/-e307. Added test for issue 1164 to t015_test.py
autotest.

Closes #1164

[codecov]

Codecov Report

Merging #1172 (9ed4ece) into develop (64094ae) will increase coverage by 0.694%.
The diff coverage is 100.000%.

❗ Current head 9ed4ece differs from pull request most recent head 5090e4d. Consider uploading reports for the commit 5090e4d to get more accurate results

@@              Coverage Diff              @@
##           develop     #1172       +/-   ##
=============================================
+ Coverage   72.298%   72.992%   +0.694%     
=============================================
  Files          229       225        -4     
  Lines        52773     52001      -772     
=============================================
- Hits         38154     37957      -197     
+ Misses       14619     14044      -575     

Impacted Files	Coverage Δ
flopy/utils/flopy_io.py	77.732% <100.000%> (+0.273%)	⬆️
flopy/mt3d/mtsft.py	6.569% <0.000%> (-72.263%)	⬇️
flopy/utils/formattedfile.py	89.032% <0.000%> (-2.581%)	⬇️
flopy/utils/mfreadnam.py	75.912% <0.000%> (-1.460%)	⬇️
flopy/modflow/mflmt.py	91.578% <0.000%> (-1.053%)	⬇️
flopy/mf6/utils/testutils.py
flopy/mt3d/mtcts.py
flopy/mf6/utils/reference.py
flopy/utils/voronoi.py
flopy/utils/mflistfile.py	68.680% <0.000%> (+0.223%)	⬆️
... and 1 more