Consider adding a replicate index to all variables about powermodels.jl

[ccoffrin]

from @rb004f, "More generally we'd also want to index by scenario, or perhaps other reasons"

Good point! It should be a fairly general way to include multiple network copies in one JuMP model. The constraints you add to link those variables together would differ based on these different applications.

[ccoffrin]

To enable this we need to, move all variable constructors to JuMP anonymous variables and add our own methods for organizing and looking up these variables. This will also will resolve the issue of how to have two Infrastructure Models in one JuMP model.

[ccoffrin]

Proposal, pm.var[:p] for looking up variables. We can also considering adding getvar and getref methods for handling various default cases (e.g. with and without variable lookup)