Add solar still ZLD unit model, costing model, test file

[kurbansitterley]

This PR adds an implementation of the ZLD solar still unit model and costing model provided by NMSU for the SETO project.

The unit model is relatively straight forward.
The most important parameter water_yield relies on user input for the weather file (.csv) and an external function in util to calculate the daily water yield. This value is used to set the model parameter water_yield.

The model assumes:

• All of the salt is removed (goes to properties_waste)
• All of the water that comes in goes out as product water (i.e., 100% recovery)

The test file tests against a case provided by the NMSU team.

The regular(non-ZLD) solar still model may be a separate PR. Documentation will also be a separate PR.

TODO

• verify source and collection steps for weather data to ensure it is properly formatted. The external utility function for calculating water yield relies on a specific organization of columns.