Pyomo update (Fixes #25)

[grgmiller]

Fixes #25
Updates the code to work with Pyomo 6.1, from Pyomo 5.6.8, so that the code is compatible with CPLEX as a solver.
The environment.yml file has been updated to require use of Pyomo>=6.0.0 and pyutilib>=6.0.0, so continued use will require updating your environment.

Specific updates include:

• pyomo elements with "Simple" in their name were renamed to "Scalar"
• For any params with string elements, needed to manually specify within=Any when defining the param in order to silence a deprecation warning
• Needed to manually specify dimen=1 for Sets that are used to index parameters when loading data
• When loading data, you can no longer specify the index as a cross product of multiple sets (e.g. index=mod.PERIODS*mod.MONTHS). Instead, they must be specified as a list of Sets (e.g. index=[mod.PERIODS, mod.MONTHS])

Other cleanup included:

• when loading data, replaced specifying multiple parameters as tuples with specifying them as lists, to speed the data loading process
• Removed PERIODS_FOR_GEN_BLD_YR, and TPS_FOR_GEN_IN_PERIOD because they are not used in the model
• changed the gen_capacity_value to elcc for Effective Load Carrying Capacity