Force limiting

Tools for force limiting.

ntfl(Source, Load, As, freq)	Norton-Thevenin Force Limit
calcAM(S, freq[, fs])	Calculate apparent mass
sefl(c, f, f0[, n])	Semi-empirical normalized force limit.
stdfs(mr, Q)	Compute the normalized force limit for simple 2-DOF system.
ctdfs(mmr1, mmr2, rmr, Q[, wr])	Compute the normalized force limit for complex 2-DOF system.