added Cython PBC#2007

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richardjgowers wants to merge 8 commits intodevelopfrom

Commits on Jul 25, 2018

added Cython pbc
richardjgowers
committed

Commits on Jul 28, 2018

use libc trig?
richardjgowers
committed

Commits on Jul 29, 2018

libc sqrt
richardjgowers
committed