Revert "We don't use pointers anywhere do we? removing matrix_cl* overload from select_events"
Revert "We don't use pointers anywhere do we? removing matrix_cl* overload from select_events"

This reverts commit bf20b7a02cf956623b9ea6e74a176561c78efd4f.
`this` is always a pointer in C++.

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