Properly implement is_linear method in Dynamics class

[tclose]

There is a method in the Dynamics class, is_linear(self), which is meant to check whether the represented dynamical system is linear (this is used in Pype9 to check whether synapses can be flattened into the cell dynamics). However, currently it only checks whether there is only one regime and does not analyse the time derivatives for non-linear functions.

Need to add a visitor that flattens all aliases in the class and then check the complete expressions of the time derivatives, as well as checking for any transitions.

[tclose]

This has been implemented in the version 1.0 release