Add an interface for the user to define expansion candidates besides what's in the input Grid

# :rocket:

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

Not necessarily essential for `v0.1`, since that seems to be able to continue under the assumption that we can restrict ourselves to expanding existing generators/transmission lines (@YifanLi86 please correct me if I'm wrong), but will be required once we want to enable Switch to choose new generators, new branches, new storage etc.

### Describe the workflow you want to enable
I wish I could provide tables for expansion candidates (generators, branches, storage), distinct from the existing grid. This information could be passed to the main user-facing function described in #62, which will further pass it to the `grid_to_switch` function which writes the Switch input CSVs. For hypothetical variable generators, we will need to grab a profile from an existing generator, so this information will probably need to flow through to the `profiles_to_switch` function as well.

### Describe your proposed implementation
The user can pass a table for each of:
- generation expansion candidates
- transmission expansion candidates (AC & DC)
- storage expansion candidates

These tables need not have _all_ of the columns of the existing tables within the Grid object, since the columns that would get interpreted by Matpower or REISE.jl are not needed, but we need all of the information required to instantiate a 'final' Grid from the Switch solution.

It would be nice if the user could also say that no expansion is allowed for one or all of these, or that expansion should only consider existing ones.

For transmission, as far as I can tell, it does not appear that we can set a limit for how much each branch can be expanded.