Add increment kwarg to model.py's run() method

[Fin109]

• Allows user to specify "increment" when running a model ex: model.run(solver=mysolver, increment=.01)

• This functions the same as setting models timeline to something that will produce a specific increment, such as; numpy.linspace(0,100,10001) (increment of .01)

[briandrawert]

This should throw an error if they both specify model.timespan and model.increment.

[briandrawert]

@jtcooper10 : raises an error if increment and self.timespan are both set... but self.timespan is always set, if the user doesn't provide a timespan then a timespan is created for them. So this approach will cause an error to be raised anytime that increment is provided, no matter what

[jtcooper10]

Timespan set in constructor if not provided by the user, causing increment to raise an error. Conversely, removing the default construction of a timespan might be considered a breaking change.