Allocations during forward map of IFGFOp

[maltezfaria]

As per the discussion with @cbauinge, it would be nice to pre-allocate all the
necessary memory for IFGF operator so that the foward map is allocation-free.
To do this, we have to:

1. Find the largest number interpolation coefficients needed at any level of the
   source tree
2. Pre-allocate two buffers of that size to keep the interpolation
   values/coefficients of two consecutive levels.

If properly done, this should allow (I think) for an allocation-free forward
map. The other advantage is that we can have an a priori upper bound on the maximum
memory required, and so will not run out of memory during the computation of a
matrix/vector product.

[maltezfaria]

Done in #33