Technical reference improvements

[adam-a-a]

Running to-do list for update technical references in readthedocs:

• update NTC to NTU in effectiveness of HX equation for VAGMD
• feed flow rate for MED-TVC should list a valid range in the first table
• feed flow rate for LT-MED can include "--" in table's valid range
• remove typo mentioning LTMED in MED-TVC reference (should say MED TVC)
• MED-TVC: fix table entries for variables including heating steam mass flow and motive steam mass flow
• Add technical reference for multiperiod VAGMD (can be used for semi-batch or batch)
• Update vagmd reference title to "vacuum air-gap" instead of "vacuum air-gapped"

[adam-a-a]

Nice-to-have running to-do:

• Add full, more lengthy documentation on LTMED, MEDTVC, and VAGMD in "background" section to provide more background explanation regarding the surrogate models
• Add custom diagrams for LT-MED, MED-TVC, and VAGMD

[kurbansitterley]

Documentation needed for:

• battery
• FPC (physical)
• multi-effect crystallizer
• deep well injection
• solar still

Updates needed for:

• Chemical softening