Adaptation of read_interpolated_variable() to the AMR

[maximegrandin]

This is an attempt to adapt read_interpolated_variable() to the AMR. The main modification is related to the case when the coordinates of several points are given, as the original procedure (sorting data according to the cellids and equating cellid and index) cannot work with the AMR.
Note: Currently (i.e., in current master version), get_cell_neighbor() searches for the neighbour cell(s) at the same refinement level as the base cell. This "works" since the correct cellid is found (no matter the neighbour's refinement level) at the imposed distance (= base-cell refinement-level resolution) from the base cell, but this does not handle correctly cases when the neighbouring cell is at a higher refinement level (i.e., we end up taking the data from the "second neighbour" cell by taking too large a step). This is not critical – it results in smoothing of the data, I think, but shouldn't in theory create artifacts – but we might want to implement a better solution at some point.

[markusbattarbee]

I think this would be a good point to add a check to exit if the variable is registered as an FSgrid variable. (At the start, check if it starts with `fg_´, redirect to read_interpolated_fsgrid_variable(), and add that dummy function with a warning of it being not yet implemented.

Interpolation in AMR space when there's a refinement level change is tricky. I'd recommend adding a check to see if the reflevels of lower_cell_id and upper_cell_id are different - if they are, continue as now but also print a warning to the user.

[markusbattarbee]

The page
https://numpy.org/doc/stable/reference/generated/numpy.argwhere.html
states that argwhere shouldn't be used for indexing, but use np.nonzero instead. Worth looking into, and could give cleaner code?

[maximegrandin]

The suggested changes have been implemented, and this has been tested.
A cleaner handling of AMR refinement boundaries is not trivial and is left for future work.

[markusbattarbee]

One more small change request:

   if name[0:3] == 'fg_':
         self.read_interpolated_fsgrid_variable(name, coordinates, operator, periodic)
         return -1

should be

   if name[0:3] == 'fg_':
         return self.read_interpolated_fsgrid_variable(name, coordinates, operator, periodic)