Give users access to underlying MELTS/thermoengine settings

[kaylai]

Add functions to allow users to set parameters for the MELTS model including model version and included phases.

[kaylai]

Working on this now. The answer is to allow the user access to the "xmlout" object that MELTS creates. I'm testing this on the "AccessMELTSThermoProperties" branch with the calculate_degassing_path() function.

Note that in order to interact with the xmlout object, users need to import thermoengine and create a MELTS instance. I plan to provide extensive documentation in a section on the ReadTheDocs covering interconnectivity with MELTS.