add model class
update example to use new model class
thermal exp, c_v, c_p, Model improvements

remove all references to main while we are at it
add error checking
fix c_p, c_v computation
add tests for model, some cleanup
fix model.density, .K, .G
add tests
remove duplicate storage of Model.avgmoduli
Merge pull request #28 from tjhei/model_class

Model class