Keyword arguments in X_A_Xt

[Nicholas Krämer]

Would be very useful to have a version of X_A_Xt(M1, M2) that does X_A_Xt(X=M1, A=M2), i.e., that works with keyword(-only) arguments.

But to do something similar to PDMats.jl, it might be advisable to keep the positional-argument-version.