Secant obeys the maxiter parameter

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Improvements to solvers at low pressure/temperature

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Moved the plotting shim functions back to CoolProp.Plots.Plots and added import to CoolProp.Plots.Plots

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>