All assumptions applied to functions of time should be transferred to their discrete versions

[moorepants]

The generation of discrete variables looks like:

        self.previous_discrete_state_symbols = \
            tuple([sm.Symbol(f.__class__.__name__ + 'p', real=True)
                   for f in self.state_symbols])

This sets a real=True assumption on each new discrete symbol but any other assumptions that may have been applied to the continuous symbol would be lost. This can have an unexpected effect when the Jacobian is calculated if some assumptions would change the output of the Jacobian wrt to any discrete symbol.

[moorepants]

A related issue is that the CSE variables don't have the assumptions of the expressions they replace. See #216 for related info.