Add a module to create profiles for solar thermal generators

[danielolsen]

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• Is your feature request essential for your project?

Describe the workflow you want to enable

Currently, for the HIFLD grid, we're excluding solar thermal generators, since we only have a profile generation process that applies to solar photovoltaic. However, if we can come up with reasonable profiles for for solar thermal, we can include them. This represents approximately 5% of the total solar capacity (1.8 GW of 37.8 GW total from EIA Form 860 for 2019).

Describe your proposed implementation

Looking at the documentation for NREL's SAM tool (https://nrel-pysam.readthedocs.io/en/master/Models.html), it seems they have several solar thermal models, of which TcsgenericSolar seems like the most generic. We should be able to use this module via PySAM in a similar way as we currently use their PV model (PVWattsv7).

[danielolsen]

Closed via #256.