nextItem alternative theta estimation options

This commit changes the next item function so that the user can specify
one of three theta estimation procedures: EAP, ML, and EAP.

This also required me to make some changes to the estimateThetaMAP
function so that the only input it took was a cat object.

I updated the documentation for both of these functions according to my
change.s
Merge branch 'CAT_KOW' of https://github.com/joshcutler/CATsurv into CAT_KOW

Conflicts:
	CAT_KOW/CATPack/R/estimateThetaMAP.R
	CAT_KOW/CATPack/R/nextItem.R
Merging commits (cont)