fix(load_node_mapping): add sim parameter to populate Mf6Splitter._model_dict

[jlarsen-usgs]

• update test_save_load_node_mapping
• fix MFSimulation.model_names, return an iterable list instead of dict keys object

[codecov]

Codecov Report

Merging #1828 (4af8133) into develop (2fa3d65) will increase coverage by 0.0%.
The diff coverage is 85.1%.

@@           Coverage Diff           @@
##           develop   #1828   +/-   ##
=======================================
  Coverage     72.1%   72.1%           
=======================================
  Files          255     255           
  Lines        56050   56092   +42     
=======================================
+ Hits         40446   40496   +50     
+ Misses       15604   15596    -8     

Impacted Files	Coverage Δ
flopy/mf6/utils/model_splitter.py	69.1% <84.9%> (+0.6%)	⬆️
flopy/mf6/modflow/mfsimulation.py	65.6% <100.0%> (+1.2%)	⬆️

[jlarsen-usgs]

PR is still in progress. I found that we need to save additional data to the JSON file for reconstruct_array() to work properly.