Start implementing support for boolean variables
Finalize support for boolean variables, remove the bool function.
Use the is_boolean_expression to determine whether the refractory period is a correct boolean expression. Also change the is_boolean_expression function to the common namespace/specififers signature
Allow "bool" as a unit in user-defined equations and make sure that it cannot be used for differential equations.
Handle subexpressions better.
Merge pull request #73 from brian-team/boolean_parameters

Allow for boolean state variables. Closes #68