Added support for converting between kex and tex, and pA and pB for the dispersion analysis.
This is performed by the new specific_analyses.relax_disp.parameters.param_conversion() function.
For this, most of the code from the assemble_param_vector() function has been shifted into
get_value(), and most of disassemble_param_vector() into set_value(). The dispersion analysis now
also has a custom sim_init_values() method to handle these parameters.
