Switch to Fourier points in FieldConvert with homogeneous expansions
This MR switches the point distribution of homogeneous expansions in FieldConvert to FourierEvenlySpaced.
The main reason for that is a performance issue recently reported when the number of Fourier modes is large. This was caused by the expansion creating a PolyEvenlySpaced distribution with many points, which scales very poorly due to the derivative matrix (for 1000 points this takes around 2 hours). This cost is also independent of the number of mpi processes...
This is also related to issue #10 (closed).