This reverts commit 6f6c3499.

This reverts commit bfc539fa, reversing
changes made to 861e5f8b.

This reverts commit 9f3863d0, reversing
changes made to bfc539fa.

Re-arrange the expressions in potential_derivatives_compute_M2P and potential_derivatives_compute_M2L to avoid over- and under-flow

# Conflicts:
#   src/engine.c

Move the (1/r)^n terms to the parent terms.

Io selection changes

Closes #651, #670, and #671

See merge request !1088