Add ability to refine grid

[je-cook]

Some eqdsks are very detailed it would be good to be able to reduce (or I suppose increase) how fine the grid is.
I imagine this will need some interpolation of some description. Along with care to keep the profiles consistent.

This will help with memory consumption on limited machines

[je-cook]

To be able to do this properly we need magnetostatics calculations, therefore going to defer to bluemira to do this for now see Fusion-Power-Plant-Framework/bluemira#3658