:bug: bc20m: Critical field at zero temperature and strain (T).

[mkovari]

bc20m This appears to be this quantity (more or less):

Effective upper critical field at zero temperature and intrinsic strain B_c2^* (0,0)

But somehow the description mentions 20 K, which I think is wrong.
process/models/tfcoil/superconducting.py

    bc20m: float
    """Critical magnetic field at 20 K and zero strain (T)."""