Implemented a specialized check_pos_definite for var matrices which first converts to a double.
Implemented a specialized check_pos_definite for var matrices which first converts to a double.

stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/stan,stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/stan,stan-dev/stan,stan-dev/math