Automatic number of bins & interpolators for off-resonance correction

[Daval-G]

As discussed in PR #180, we should provide at some point an option in the off-resonance correction operator to automatically suggest a number of bins and interpolators for a given B0+R2* field map. The default options should also be based on their error in matrix factorization in the case of a typical 3T or 7T B0 field map.