Expand investment cost input options to allow one per plant per investment year

# :rocket:

- [ ] Is your feature request essential for your project?

### Describe the workflow you want to enable
Currently, the investment cost input to Switch is a single value per generation type for the entire country.
Big picture: we want to develop a workflow that can modify any or every investment cost value for each plant's generation type at each bus around the country.
This is step 1 of that process: allowing for an optional override process while also ensuring that each plant has an investment cost value to send to Switch.

### Describe your proposed implementation
Adds an optional parameter input to grid_to_switch.py that could be a Series, Dictionary, or DataFrame.
That optional parameter is passed to the internal function build_gen_build_costs.

The new parameter has
Index plant_id
If it is a single column, it is assumed to apply to all investment years.
If it has multiple columns, then each column is assumed to map to a particular investment year and should be labeled as such
Likely updates via the section "Create a dataframe from the collected lists," currently lines 335-343
One note: Storage devices will need an Energy capital cost value and a Power capital cost value as part of the input structure.

### Describe alternatives you've considered, if relevant
None at this time.

### Additional context
There is an open question of how to handle hypothetical plants that don't correspond to original plants: see issue #69