Add parameter validation to equations of state

Conflicts:
	burnman/equation_of_state.py
This is now out of place, so remove it.  This type of error checking should be done in validate_parameters()
Probably better not to assume values, just use nan in this case
Fix tests
Augment warnings
Remove superfluous set_method
Merge pull request #57 from ian-r-rose/validate_parameters

Validate parameters