update(MFSimulationBase): allow simulations to have no attached models

[jlarsen-usgs]

Adding this PR because:

• we should be able to define time discretization for a simulation structure prior to adding models and packages
• models are a seperate entity that underly a simulation object
  • IMS and TDIS are the only simulation based packages
    *a simulation object should be able to be written without a model attached to it
  • model objects should be able to be read with no packages attached

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 73.1%. Comparing base (0f14f1f) to head (a4bdd96).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2140     +/-   ##
=========================================
- Coverage     73.1%   73.1%   -0.1%     
=========================================
  Files          260     260             
  Lines        59491   59495      +4     
=========================================
+ Hits         43492   43494      +2     
- Misses       15999   16001      +2     

Files	Coverage Δ
flopy/mf6/mfmodel.py	60.4% <50.0%> (-0.1%)	⬇️
flopy/mf6/mfsimbase.py	67.9% <50.0%> (-0.1%)	⬇️