User sees "error" printed during run_MCMC. Infection history swap proposal indexing error

[jameshay218]

When an individual is only "alive" for 3 time periods, then the default move_size argument (also 3) in mcmc_pars of run_MCMC appears to incorrectly place infections before the time point of birth. This is fixed when move_size is set to 2, suggesting that the problem is when the swap step overshoots and has to account for the boundary effect. Need to fix this in the infection history proposal function.