Revisions to ST_Union for raster: get rid of dependency on rastexpr (and get rid...

Revisions to ST_Union for raster: get rid of dependency on rastexpr (and get rid of type), put in regress tests, get rid of many ST_Union permutations and limit to st_union(rast), st_union(rast,p_expression), st_union(rast,band_num), st_union(rast,band_num,p_expression).  Note for drop I'm not dropping those other versions in case people have installed them from scripts folder or are currently using them.  just dropping the ones I'm replacing.

git-svn-id: http://svn.osgeo.org/postgis/trunk@8351 b70326c6-7e19-0410-871a-916f4a2858ee