Different interpolation options for different variables

[NicWayand]

Allows better control of interpolation of different variables when dealing with sparse forcing data.

[NicWayand]

For future reference when slack history is gone...

@Chrismarsh says:

Currently interpolators are store on a per-triangle basis to allow for parallelizationn and are initialized on a per module basis (look at the interp_met stuff). I did this as I figured this would come up eventually.

I would have a map that stores an interpolator type associated with a module in global. Currently modules query global_param->interp_algorithm to get the the type of interpolator needed, so it should be easy to slip in. interp.init can still ask global wtf it should use, and global can tell it mand the module remains unaware of what it is using other than "global told me what to do"

I think a simple refactor of global_param->interp_algorithm to global->get_interp_alg() would suffice. It looks up what's needed and off it goes.

All the config heavy lifting is done so you can use per-module configuration to override the global interp