Update notation

Address issue #88

Update convolution to have a minus instead of plus. Update Legendre
example documentation. Change xi to eta. Added notation note in
introduction to explain the notation with convolutions.
Merge remote-tracking branch 'upstream/master' into issue-88-basis-documentation

Conflicts:
	notebooks/cahn_hilliard.ipynb
	notebooks/cahn_hilliard_Legendre.ipynb
Fix notation

Address #88

Some locations still using \xi instead of \eta.
Merge pull request #120 from wd15/issue-88-basis-documentation

Issue 88 basis documentation