Baseline indexer part1

[jbeilstenedmands]

Adds the first part of a baseline c++ indexer implementation (doing a fft of the strong spots and finding a list of candidate basis vectors).

Requires dx2 to be added as a subdirectory (this branch: dials/dx2#1).

Can be run as e.g:

./bin/baseline_indexer \
  -r /dls/mx-scratch/jbe/test_cuda_spotfinder/cm37235-2_ins_14_24_rot/strong.refl \
  -e /dls/mx-scratch/jbe/test_cuda_spotfinder/cm37235-2_ins_14_24_rot/imported.expt \
  --max-cell 100 \
  --dmin 1.81

Refinement and scoring of basis vector combinations will come as a separate PR once this is merged in.

For now, just for single-sweep, single-panel detector to get started.

Max cell and dmin currently required inputs, pending inclusion of annlib-style code.