refactor(Modpath7.create_mp7): expose porosity parameter of Modpath7Bas

[martclanor]

This PR exposes the porosity parameter of Modpath7Bas in Modpath7's convenience method, create_mp7. This idea is raised in #2339.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.9%. Comparing base (5555d15) to head (5a82635).
Report is 12 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2340     +/-   ##
=========================================
- Coverage     74.3%   73.9%   -0.5%     
=========================================
  Files          294     292      -2     
  Lines        59562   59715    +153     
=========================================
- Hits         44282   44142    -140     
- Misses       15280   15573    +293     

Files with missing lines	Coverage Δ
flopy/modpath/mp7.py	86.5% <100.0%> (ø)

... and 25 files with indirect coverage changes