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
add multi_gp_cholesky syntax tests
merge alyst changes
merge changes
fix merge conflict
Merge branch 'develop' into feature/multi-gp-cholesky
Merge pull request #808 from stan-dev/feature/multi-gp-cholesky

Feature/multi gp cholesky