Adjusted element_prod and element_div for vectors: Temporaries are handled in op_executor

Previously the temporary dispatch was done directly in element_prod() and element_div().
This is prohibitive for using the scheduler efficiently (temporaries have limited lifetime).
Scheduler: Renamed OPERATION_BINARY_ELEMENT_MULT_TYPE to OPERATION_BINARY_ELEMENT_PROD_TYPE

This is more uniform with element_prod()
Scheduler: Added support for elementwise vector operations.