FMM for UnregularizedLayerPotentialSource

This adds an FMM execution backend to UnregularizedLayerPotentialSource, based heavily off of QBXLayerPotentialSource.

Some of the QBXLayerPotentialSource code responsible for preprocessing the FMM is refactored into LayerPotentialSourceBase.

added 5 commits

• 1c6b9917 - Whitespace fix.
• e78cc1eb - Fix typo.
• 828860c8 - Move some FMM preprocessing code to LayerPotentialSourceBase.
• c660125d - Finish implementing FMM based execution in UnregularizedLayerPotentialSource.
• 29ff7b30 - Get the FMM working; add an off surface eval test.

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unmarked as a Work In Progress

changed the description

added 34 commits

• 29ff7b30...292086df - 33 commits from branch master
• 0ca64a27 - Merge branch 'master' into unregularized-fmm

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The tests pass modulo #60 (closed)

resolved all discussions

This now has a merge conflict. Could you take a quick look?

added 10 commits

• 0ca64a27...697eaff3 - 8 commits from branch master
• 4fa04cf5 - Merge branch 'master' into unregularized-fmm
• 75c6a79e - Change some terminology:

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This now has a merge conflict. Could you take a quick look?

Done. Also an attempt at improving terminology: 75c6a79e

mentioned in commit 438a85e0

merged

LGTM. Thanks!