add documentation and topo shielding factor param
improve doc and add pressure term to interp fn
fix issues with multiglaciate and in-place updates

In-place updates causes problems with using the model with PyMC.
Fix that and simplify the code a bit.
remove redundant dict entry
tick up version 0.3.0
Merge pull request #13 from zploskey/master

merge 0.3.0