First Draft of Creep Parameter Fitting
Implement routine to calculate the "internal variable" , xi in SAND97-2601 Eqn, 2-27
ODE

implemented ODE solver.
Merge branch 'Routine-for-fitting-Creep-Parameters'
C-S model solved for lateral and axial strains using CS_DE.R.  The model still does not match results from Callahan.
Merge branch 'Routine-for-fitting-Creep-Parameters'
CS_DE-01.R is the first successful version to solve the creep strain rate differential equation.
CS ODE runs

Current state it works for one test at a time, next implement it for
all the tests