update(styles): add graph_legend fontsize parameters

[jlarsen-usgs]

• add fontsize argument to graph_legend_title
• add fontsize and title_fontsize kwargs to graph_legend

[codecov]

Codecov Report

Merging #1702 (7f12e1b) into develop (ea1f5f0) will increase coverage by 0.0%.
The diff coverage is 16.6%.

@@           Coverage Diff           @@
##           develop   #1702   +/-   ##
=======================================
  Coverage     71.9%   71.9%           
=======================================
  Files          253     253           
  Lines        55890   55890           
=======================================
+ Hits         40190   40204   +14     
+ Misses       15700   15686   -14     

Impacted Files	Coverage Δ
flopy/plot/styles.py	27.4% <16.6%> (ø)
flopy/mbase.py	69.1% <0.0%> (+0.1%)	⬆️
flopy/export/shapefile_utils.py	90.5% <0.0%> (+0.2%)	⬆️
flopy/utils/mfreadnam.py	33.4% <0.0%> (+0.2%)	⬆️
flopy/mf6/mfbase.py	82.1% <0.0%> (+0.2%)	⬆️
flopy/utils/flopy_io.py	78.2% <0.0%> (+0.4%)	⬆️
flopy/utils/get_modflow.py	63.9% <0.0%> (+2.1%)	⬆️