fix multi_gp_log() signature in doc
sync multi_normal and multi_normal_cholesky

rearrange the code so that it is more evident which code parts
are shared and which are different between multi_normal and
multi_normal_cholesky distributions
use dedicated variable for Cholesky factor in tests

don't reuse Sigma variable to store its Cholesky factor
cleanup multi_gp

remove unused headers and 'using' directives,
update template parameter names
add multi_gp_cholesky distribution
Added cbind functions and tests
Added rbind functions and tests
Extra test to cbind and rbind
add multi_gp_cholesky syntax tests
cbind: functions with correct promotion of args and added agrad-rev tests
rbind: functions with correct promotion of args and added agrad-rev tests
Some basic fwd-rev tests for cbind and rbind
Added cbind and rbind to function signatures and related signature tests
Minor improments to rbind and cbind
Fix incorrect reference to inverse in to_matrix function signature test
Added documentation for cbind and rbind + minor fix for Transposition Postfix Operator documentation
split out parser_test
Fixed typo in rbind documentation as was pointed out by Peter Li
Merge branch 'develop' into feature/issue-787-cbind-rbind
Renamed cbind and rbind to append_col and append_row, respectively