Gravity speedup

@pdraper I pushed this to my branch instead of a separate branch for you to inspect. Is that useful to cherry-pick to master ?

Added 115 commits:

• 23b2fff2...55d562a1 - 114 commits from branch master
• eb07db4d - Merge branch 'master' into gravity_speedup

Added 1 commit:

• 1b1a523d - Call the correct function for the M2L derivative checks.

Status changed to merged

mentioned in commit bbc990c1