Add support for log-posterior emulators in MCMC functions/sim study pipeline

[arob5]

• Add argument to MCMC functions that indicates the the llik emulator is actually a log posterior emulator.
• Or alternatively add a flag to llikEmulator objects, so don't have to worry about passing this information in.
• Need to add adjustments to llikEmulator bound constraints to account for the fact that the bound will vary due to the prior. This may cause some issues since the bound is no longer a constant - it will depend on the input.