dd_bcast implemented in shmem

See routine comments for implementation details.
Tested with 2 and 4 p.
Fixed dd_sendrecv_rvec

dd_sendrecv_rvec caused problems in one of the pdb2gmx regtests
due to incorrect size of data passed to shmem_float_put.
This is fixed here.
The dd_bcastc routine has been added as well.

Tested with 2 and 4 procs