Add DPmem and collapsed conjugate XRPs

[ngoodman]

First add these in probjs (where i will open an issue) then bind through to webchurch.

[longouyang]

I added DPmem in 3675ba9609d34c1fd047d15087d3dcb6e32b4280 but only as a builtin function that manages a CRP through the use of multinomial, rather than as an XRP in its own right