Add prefix() and postfix() methods to Equations class

[nishantraghuvanshi]

This PR fixes issue #1050, adding prefix() and postfix() methods to the Equations class.
These methods allow users to rename all variable names in a set of equations, which is useful for avoiding name conflicts in multi-population models.

Example

eqs = Equations('''
    dv/dt = -v/tau : volt
    I = g*E : amp
''')

exc_eqs = eqs.prefix('exc_')  # Variables become: exc_v, exc_I
inh_eqs = eqs.postfix('_inh')  # Variables become: v_inh, I_inh

Key Design Decisions

• Recursive behavior: Methods apply to current variable names, so eqs.prefix('a_').prefix('b_') produces b_a_v (nested)
• Comprehensive validation: Full Python identifier validation with keyword checks
• Built-in protection: Variables like t, dt, xi, i, N are never renamed
• Scope: Only explicitly defined variables are renamed (diff eqs, parameters, subexpressions), not external references

Please suggest any changes that I need to make in the above implementation.
@thesamovar @mstimberg

[mstimberg]

Hi @nishantraghuvanshi, thank you for the PR. This issue is a good way of getting familiar with Brian's code and our contribution process, but please be aware that we most likely won't merge it. We should have probably closed #1050, since our discussions since then have converged onto a more general solution (see #1244). So, I'd be happy to continue to work with you on this PR, but I also understand if you want to focus on things that will actually make it into Brian's codebase.

[nishantraghuvanshi]

Thanks for the transparency, @mstimberg
I really appreciate you pointing me toward #1244.

I've started reading through the issue thread (it's quite long, so I'm still digesting everything), but I can already see the templating approach with {...} syntax is much more flexible than the simple prefix/postfix methods.

A few questions as I'm learning about this:

1. From what I've read so far, it looks like the template approach can handle the prefix/suffix use case (by doing things like {name}_{suffix}). Is that right, or am I understanding it incorrectly?

2. How far along is Structured equations with equation templates #1244? Is there a specific area where I can help?

I'm definitely interested in contributing to something that's going into Brian2. Whether that means pivoting to help with #1244 or continuing with this PR as a learning exercise to better understand the codebase, I'm excited to contribute wherever makes the most sense. Would love your guidance on the best path forward.

[mstimberg]

Hi @nishantraghuvanshi. I will close this PR, but if you are still interested in contributing to #1244, please make a comment over in that PR. I haven't looked into that branch in quite some time, so I'm not exactly sure how much work it still needs (also, for now, it does not apply cleanly to master).