implement FOD(F)

4 functions:

• convolve_watson
• convolve_bingham
• convolve_deltas
• convolve_SH

Each function takes as input:

• compartment
• all acquisition parameters required by compartment (should include bvec)
• fODF parameters
  • for non-SH: list of tuple (volume fraction, fibre orientation / fibre orientation + kappa) for example: crossing = [(0.3, 'delta', [1., 0., 0.]), (0.7, 'watson', ([0, 1, 0], 4.)]

def convolve_crossing(compartment, acquistion, crossing):
   for volume_fraction, dtype, param in crossing:
       func = {'watson': convolve_watson, 'delta': convolve_delta, 'bingham': convolve_bingham}[dtype]
       signal += volume_fraction * func(compartment, acquisition, params)

Output is the signal as 1D array

Sub-issues:

• Add fODF to GUI #63
• write convolve_SH #59
• write convolve_crossing & convolve_delta #60
• write convolve_watson #61 (closed)
• write convolve_bingham #62