AIS uses rmh instead of lmh

AIS was setup primarily for BOPP for which using rmh instead of lmh is essential to move to regions with negligible mass under the prior. This change was made into the BOPP code but for some reason the version that went into Anglican did not have the change. This is thus just a two character to change to use rmh instead.

This has also made me realise that we never got rid of the horrible proposal hardcoding in rmh...