added the velocity functionality to rhomboid lb-boundary

I added the velocity option to the lbboundary rhomboid (as was done for the wall previously) so that we can use it to start fluid at the beginning of the channel instead of setting it individually in tcl for each lbnode in each time step. I did not do it for sphere, cylinder, etc. because I did not know if there was some reason for not specifying it for all these other boundaries in the first place. 
Please, merge. Thank you.
Iva Jancigova (object-in-fluid)