Add 2D elasticity notebook example

Address issue #68

Add first draft of elasticity notebook. Still need to work on the api for random microstructures for elasticity for more than 2 phases. Need to know what units Sfepy expects the Young's modulus to have.
Merge branch 'master' into elasticity-notebook
Add re-sized coefficients to 2D elasticity notebook

Address issue #68

Add re-sized coefficients to 2D elasticity notebook to demonstrate how
to use the method resize_coeff.
Add MKS equation derivation

Address issue #65

Add MKS equation derivation to show where the MKS equation comes from.
Correct typos in the MKS derivation.

Address issue #65

Correct typos in the MKS derivation
Merge branch 'elasticity-notebook'

Fix #68, fix #65