[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.
Merge pull request #144 from blinkseb/mcmc_fix

[MCMC] Dynamic burn-in has no effect