Make ramp rate input flexible to other grids

# :beetle:

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### Bug summary
Currently, generator ramp rates provided in a grid's `plant.csv` file are overridden in `REISE.jl`'s `prepare.jl` file, regardless of grid type, to equal a value according to a generator's capacity and fuel type (see [here](https://github.com/Breakthrough-Energy/REISE.jl/blob/develop/src/prepare.jl#L25-#L45) for the referenced code). This was originally implemented as a means to improve the ramp rate specifications for the generators in the TAMU U.S.A. grid. However, since we are trying to make the ecosystem more flexible to other grid types, we should make this TAMU-grid-specific adjustment only apply to the TAMU U.S.A. grid. Additionally, this adjustment probably is better suited for inclusion in `PowerSimData` than it is in `REISE.jl`.