Feature/interpolator

This MR introduces an Interpolator class which adds several features/improvements over how interpolation was handled by the PtsField class:

* The brute force search algorithm was swapped for boosts rtree algorithm. This change decreases the interpolation time from hours to seconds
* Besides the modified shepard and linear/quadratic interpolation, two new algorithms, nearest neighbor and Gaussian smoothing were added
* The new class can interpolate between ptFields, ptsFields and expansions and expansions. The latter is only useful when dealing with different meshes.
* Several bugfixes

FieldConvert and EquationSystem were ported to this new class to benefit from the speedup.

This MR also includes the changes what were originally in !582

The required boost version was increased to 1.56 as this is the first version that includes geometry::rtree which works on windows

See merge request !584