Gray mechanical threshold stress

This is a generalisation of Gray's model, allowing parameters to vary with density.
type name
mts
parameters
mu0(rho)	shear modulus (function of rho)
d0(rho)		thermal softening (function of rho)
t0(rho)		thermal softening scale temperature (function of rho)
sigma_a(rho)	athermal flow stress (function of rho)
b(rho)		Burgers vector (function of rho)
a0 a1 a2	coefficients in theta0 equation (real)
kappa		work-hardening exponent (real)
g0(rho) g0s(rho)	Peierls factors / Boltzmann's constant (functions of rho)
epsdot0 epsdot0s	characteristic strain rates (functions of rho)
sigmahat0s	work-hardening stress (function of rho)
p q		powers in plastic rate terms

All functions are WXMATHS class Realfunction1d *