CH simulation can use odd grid point count.

Issue #84

Previously the Cahn-Hilliard simulation was broken for an odd grid
point count. The code correctly assembles the k array so that even
arrays are (0, 1, -2, -1) for N=4 and odd arrays are (0, 1, 2, -2, -1)
for N=5. This has not been rigorously tested yet and a test case is
required as well as changing the notebooks to use odd N counts.
Update notebooks to use odd N count.

Address #84

Notebooks use odd N count as MKS is generally used on odd grid count.
Merge pull request #118 from wd15/issue84

Fix Issue #84