Comprehensive support for MODFLOW-USG ("USG Transport" version) (#2575)

* fix issues related to old temporal reference

* fix test errors in flow problems, and start working on transport tests

* Resolve conflits arose from merge changes to mfusg #1

* Resolve conflicts arose from changes mfusg

* Resolve all conflicts arise from mfusg changes made by hzhang4

* Ruff linting ...

* linting with Ruff ..

* use flopy_oi in mbase

* Address PR comments and use relpath_safe

* Run Ruff after addressing  PR comments

* minor Ruff linting ..

* Remove redundant files and apply minor Ruff linting ..

* Remove redundant files ..

* remove docx file ..

Author: aymanalz

.docs/Notebooks/gridgen_example.py

@@ -426,7 +426,7 @@
     structured=False,
 )
 disu = flopy.mfusg.MfUsgDisU(m, **gridprops)
-bas = flopy.modflow.ModflowBas(m)
+bas = flopy.mfusg.MfUsgBas(m)
 lpf = flopy.mfusg.MfUsgLpf(m)
 chd = flopy.modflow.ModflowChd(m, stress_period_data=chdspd)
 sms = flopy.mfusg.MfUsgSms(m)