[OPTIQ-421] Add COVAR_POP and COVAR_SAMP aggregate functions

Close apache/incubator-optiq#14

Pull requests closed by other recent changes (committed already):
Close apache/incubator-optiq#12
Close apache/incubator-optiq#5
Close apache/incubator-optiq#2
Merge remote-tracking branch 'upstream/master'

Conflicts:
	core/src/main/java/org/eigenbase/sql2rel/StandardConvertletTable.java
Merge Fixes for OPTIQ-421