Merge pull request #10 from SalomonKoen/master

Yo, im pullin a request :)
Update the derivation of Crank-Nicolson model.

Includes changes to indexing notation to avoid
conflict with "i" as used in stability analysis
for complex numbers. The non-linear source term
is handled better.
Implement solver for system using CN model.
Merge pull request #11 from MichaelHatherly/mh/index_changes

Update the derivation of Crank-Nicolson model.
Merge pull request #12 from MichaelHatherly/mh/implement_cn_solver

Implement solver for system using CN model.