Better checking/handling of custom circuit string definitions about impedance.py HOT 2 CLOSED

While we're in there validating the circuit, we should decide if we want to enforce strict numbering - i.e. does R2 have to follow R1? is R1-R3-R2 acceptable, or even R1-R3?

from impedance.py.

Regarding @nealde's comment, I think different groups have different standards of naming their elements. I tend to name mine elements grouped in parallel units since R1 and C1 correspond to one electrochemical process and R2/L2/C2 correspond to some other process, so all the 1s go together and all the 2s go together.

R0-(R1/C1)-(R2/L2/C2)

but you could certainly name it

R0-(R1/C0)-(R2/L0/C1)

I think as long as each element is uniquely named, I would like to err on the side of giving the user flexibility in defining how they want to name their circuit elements.

from impedance.py.