add st_simulate option to list settable parameters

[jdpipe]

The class solartherm.simulation.Simulator reads and modifies the _init.xml files used by the modelica executables. These files contain XML describing the parameters and variables in the simulation, including the descriptions as extracted from the Modelica code.

By using the XML XPATH features, it should be possible to quickly and easily list out the parameters that are available for modification by the user, (isValueChangeable="true") eg

st_simulate --list-params SimpleSystem.mo

This would compile the model (or bypass) which creates the _init.xml file, then read the file and check for suitable variables. It would then output the names of those parameters, similarly to

st_plotmat -n