FIX: _eqcalculate: Rename models -> model for backwards compatibility

[bocklund]

In reviewing #270, I overlooked the fact that calculate takes the argument model, which a caller could pass as a keyword argument to _eqcalculate. #270 changed _eqcalculate to explicitly take a models argument and pass it to calculate.

This preserves the behavior from #270 and stays backwards compatible if anyone was using model as a keyword argument that got passed through to calculate.