Allow ConditionalDerivedVariables to be used in DerivedVariable expressions

[pgleeson]

Currently variables defined in a ConditionalDerivedVariable expression cannot be used in a DerivedVariable expression.

Example where this fails (uncomment line for DerivedVariable iMemb):

https://github.com/OpenSourceBrain/FergusonEtAl2014-CA1PyrCell/blob/341fef9d14a10e64688838a85b02950b7ea3af1e/NeuroML2/IzhikevichFerguson.xml

[pgleeson]

@robertcannon if you have any insight into why this should be the case it would be great. A DerivedVariable can certainly depend on/use other DerivedVariables, and ConditionalDerivedVariable can probably depend on DerivedVariables, but is there any fundamental reason why a DerivedVariable can't use a variable defined in ConditionalDerivedVariable?

[robertcannon]

I don't think there is any fundamental reason and I can't see anything in the jLEMS code that treats them differently either. So I'll have to try that example to see where it is failing.