Nearest neighbor interpolation

Added nearest neighbor interpolation using scipy.kdtree as well as a number of fixes and performance improvments.

Interpolation coefficients are now memoized

v0.3.3

append fixes

v0.3.2

fixed for new cell_tree2d API

v0.3.1

fixed meta.yaml

Cell Location, Index Translation, Interpolation, and Memoization

0.2

copy parameter added to signal for read-only results