Crank nicholson#36

Draft

brmather wants to merge 3 commits intodevfrom

crank-nicholson

Commits on Jul 1, 2020

preliminary crank nicholson scheme
Ben Mather
committed

Commits on Jul 2, 2020

enforce BCs and fix timestep size
Ben Mather
committed

Commits on Jul 8, 2020

improvements to area weights - still not perfect..?
Ben Mather
committed