Coordinate index access in input file?

[mikekryjak]

This PR will go a long way towards making 2D production simulations of Hermes-3 way more user friendly:
#3222

Another thing that would be super helpful is to be able to access indices in the input file. This would be very helpful to set sources at boundaries, for example.

[ZedThree]

This would be trivial -- the hardest part would be agreeing on appropriate names!

[mikekryjak]

At the risk of sounding a bit cynical, I think we are past the point of appropriate names since we already have three definitions of x and y: the domain coordinates, the input file coordinates and the index space.

I would suggest x_index and y_index.