[sherholz]

This refactors and adds comments to the split-and-merge (SaM) fitting code for the parallax-aware von-Mises Fischer mixture models (PAVMM).

In addition to adding comments to the code, this PR adds the following change/refactoring:
SaM - partial updating:

• When doing a partial update after splitting a PAVMM component, the sufficient statistics are updated during the partial update. Previously, they were copied and not updated.
• It is now possible to use the current/previous sufficient stats as prior during the partial update.
  Note: This option is currently not used.
  Merging:
• Merging two components can now check the split statistics to check if the merged components would potentially trigger a split in the next iteration when being merged.
  Note: This option is currently not used.
  PAVMM-EM:
• M-steps:
  • The prior sufficient statistics from previous runs are now merged with the current sufficient statistics before doing the M-steps for the different PAVMM parameters.
  • This allows us to remove the prior statistics from previous iterations from the M-step functions
  • Partial updates are now also merging the previous statistics with the partial updated ones.
  • This allows us to reuse the same M-step function as used for the standard update steps, and we can remove the partial M-step functions