Adapt the generate_beta function to take two parameters

[MJomaba]

Allow the function to take two parameters instead of one, if the second parameter is not specified, used the current reduction.

[MJomaba]

@johnlees

[johnlees]

What are the two parameters needed? Currently we have an initial beta, a reduction factor, and a period over which the reduction occurs

[MJomaba]

We might everything we want already then. I guess it would be better to be able to specify beta1 and beta2 (so before and after) or beta 1 and then reduction (as it is now).

[johnlees]

I can change that. Do you have a sensible beta start and beta end value to use as default?

[johnlees]

This function is used in the grid search and MCMC to generate the beta range from a start point. We should generalise this to make it a closure in these cases, so the user can use different parameters for effect and period of reduction