Be able to directly specify final latex representation of identifiers

[subsurfaceiodev]

Currently, if a have a function with an argument like effective_stress which in geotechnical engineering is represented as

there is no way to specify directly its latex representation using identifiers = dict(effective_stress="σ_v'") because, of course, it throws ValueError: 'σ_v'' is not an identifier name.

It would be great to simply specify the final latex representation desired for this type of special parameters.