Add bulk velocity as moments to output

[jmahlmann]

One can add bulk velocity components with the typical syntax to output fields, e.g., Vx_1 for the bulk velocity in the x-direction for species 1.

[haykh]

@jmahlmann could you please test this. from the limited set of tests I've done, it seems to work ok. in the input file you can now specify V_1_2 or simply V (just like with other moments) to output the bulk 3-velocity (it outputs all components, so behavior is similar to fields).