Add DMC config module to MC valid

[syhwawa]

• Fixes MC Failed with newly added DMC module.  #70
• Fixes FileNotFoundError when matsim_overrides.json File is Missing #72

Regarding to the issue #70, MC failed due to a validation step when testing DMC module in the simulation.

Here is the DMC module we would like to try in the matsim config:

  <module name="DiscreteModeChoice">
    <param name="modelType" value="Tour" />
    <param name="selector" value="MultinomialLogit" />
	<parameterset type="selector:MultinomialLogit" >
	<!-- Defines whether candidates with a utility lower than the minimum utility should be filtered out. -->
	<param name="considerMinimumUtility" value="false" />
	<!-- Candidates with a utility above that threshold will be cut off to this value. -->
	<param name="maximumUtility" value="700.0" />
	<!-- Candidates with a utility lower than that threshold will not be considered by default. -->
	<param name="minimumUtility" value="-700.0" />
    </parameterset>
   <param name="tourConstraints" value="SubtourMode" />
   <!--		<param name="tourConstraints" value="VehicleContinuity,SubtourMode" />-->
   <param name="tourFinder" value="PlanBased" />
   <param name="tourEstimator" value="Uniform" />
   <parameterset type="modeAvailability:Car">
	<param name="availableModes" value="car,pt,walk,bike" />
  </parameterset>
  </module>

In the base.py file, it seems when building parameters key process, it cannot handel inputs selector:MultinomialLogit and modeAvailability:Car in the DMC module with error below:

  File "/Users/yuhao.sun/Personal_projects/mc/mc/base.py", line 147, in build_from_xml
    _, key, uid = build_paramset_key(sub_object)
  File "/Users/yuhao.sun/Personal_projects/mc/mc/base.py", line 998, in build_paramset_key
    raise ValueError(
ValueError: Cannot build paramset key due to unrecognised parameterset: selector:MultinomialLogit in xml

    self.build_from_xml(xml_object)
  File "/Users/yuhao.sun/Personal_projects/mc/mc/base.py", line 147, in build_from_xml
    _, key, uid = build_paramset_key(sub_object)
  File "/Users/yuhao.sun/Personal_projects/mc/mc/base.py", line 998, in build_paramset_key
    raise ValueError(
ValueError: Cannot build paramset key due to unrecognised parameterset: modeAvailability:Car in xml

I have added new conditions to handle this type of input in the build_paramset_key function to the base.py.

And updated mc work fine with newly added DMC config when I test the matsim config locally using mc debug cli.

---

Regarding to the issue #72, I have also updated the load_overrides function in autostep.py to check for the existence of the file before attempting to open it.

[mfitz]

The fix looks reasonable, but I don't see any unit tests for the new code, and sure enough, when I run the code coverage report for this branch and for main locally, I see that coverage has dropped from 85.94% to 85.61% with your change. We should be testing any new code we add. We need to add tests for this new logic.