Current PBP MH is parallel, should be single chain

[tlienart]

in https://github.com/tlienart/EPBP/blob/master/lib_pbp.jl#L28-L40

• should be a single chain from which N samples are drawn instead of $N$ chains which generate samples
• does not change discussion on complexity

[tlienart]

Note that original code from Ihler and McAllester does this (parallel chains) potentially because it allows better control of # of iterations + comp cost