feat(PRT): add PRT package classes

[wpbonelli]

• add DFN and class files for MF6 particle tracking models

[codecov[bot]]

Codecov Report

Merging #1921 (01f93db) into develop (0418f85) will decrease coverage by 0.1%. The diff coverage is 56.2%.

@@            Coverage Diff            @@
##           develop   #1921     +/-   ##
=========================================
- Coverage     72.6%   72.5%   -0.1%     
=========================================
  Files          257     267     +10     
  Lines        57585   57800    +215     
=========================================
+ Hits         41835   41950    +115     
- Misses       15750   15850    +100     

Files Changed	Coverage Δ
flopy/mf6/modflow/mfnam.py	100.0% <ø> (ø)
flopy/mf6/modflow/mftdis.py	100.0% <ø> (ø)
flopy/mf6/modflow/mfprt.py	41.1% <41.1%> (ø)
flopy/mf6/modflow/mfprtdis.py	46.4% <46.4%> (ø)
flopy/mf6/modflow/mfprtdisv.py	46.4% <46.4%> (ø)
flopy/mf6/modflow/mfprtprp.py	47.6% <47.6%> (ø)
flopy/mf6/modflow/mfgwfprt.py	53.8% <53.8%> (ø)
flopy/mf6/modflow/mfpts.py	54.5% <54.5%> (ø)
flopy/mf6/modflow/mfprtnam.py	56.2% <56.2%> (ø)
flopy/mf6/modflow/mfprtoc.py	61.5% <61.5%> (ø)
... and 4 more

... and 1 file with indirect coverage changes