[MCMC] Dynamic burn-in has no effect

MCMCCalculator calls SetConfidenceLevel on MCMCInterval when creating it.
SetConfidenceLevel calls DetermineInterval, which compute the interval from the Markov Chain
for a given confidence level. This results is cached, so if we change the number of burn-in steps
after, it'll have no effect.

This PR fixes this issue by cloning  the MCMCInterval to reset its state, and sets the number of burn-in steps *before* calling SetConfidenceInterval