nvar property returns length of state variables

[geeythree]

This PR resolves issue #549

Previously the Models could specify incorrect _nvar.
The nvar property is now updated and returns the length of state_variables.

Existing behavior:

class Model:
    _nvar = 3
    state_variables = 'r', 'V'

m = Model()
m.nvar()  != len(m.state_variables)

New behavior:

class Model:
    state_variables = 'r', 'V'

m = Model()
m.nvar()  == len(m.state_variables)

[geeythree]

Hello, I am new to this community and would love to engage and contribute further. As I am still learning the TVB code base, I had chosen issue #549 to familiarise myself with the same. I request the maintainers to kindly go through the PR. 😃

[maedoc]

Thanks for your contribution. Hope to see you on some of the other issues 👍

[maedoc]

@geeythree can you additionally go through the different model classes and remove their custom _nvar defintions. For instance, this line can be removed.

Also note the rateML generated models currently generate an _nvar attribute which can also be removed.

[geeythree]

@maedoc Sure, I can do that. Shall I update these changes in the same PR?

[maedoc]

Yes, please just push more commits.

[maedoc]

closing for #720 which has the rest of the changes requested