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[bot]]

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

```diff @@ 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](https://app.codecov.io/gh/modflowpy/flopy/pull/2140?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy) | Coverage Δ | | |---|---|---| | [flopy/mf6/mfmodel.py](https://app.codecov.io/gh/modflowpy/flopy/pull/2140?src=pr&el=tree&filepath=flopy%2Fmf6%2Fmfmodel.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy#diff-ZmxvcHkvbWY2L21mbW9kZWwucHk=) | `60.4% <50.0%> (-0.1%)` | :arrow_down: | | [flopy/mf6/mfsimbase.py](https://app.codecov.io/gh/modflowpy/flopy/pull/2140?src=pr&el=tree&filepath=flopy%2Fmf6%2Fmfsimbase.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy#diff-ZmxvcHkvbWY2L21mc2ltYmFzZS5weQ==) | `67.9% <50.0%> (-0.1%)` | :arrow_down: |