diagnostic for second london equation

[RevathiJambunathan]

We need a diagnostic that computes curl_of_Js = -(1/lambda^2.mu)B to show that we satisfy the second london equation

[ajnonaka]

EvolveBLondon computes B = (1/lambda^2 mu0) curl(J) and optionally writes to a plotfile.
I think this should be mu, not mu0 here.