[microfix] add a condition for cdim to be 3 to write the shift in GK field 3x2v

[Antoinehoff]

The 2x2v GK regression test are segfaulting since the merge of the write twist-shift function because if cdim==2, the bc_sheath updater is not allocated so gk_field_2x3x_write_twistshift is looking for unallocated arrays.

This PR just add condition for cdim==3. We could also remove the condition and make gk_field_2x3x_write_twistshift to be deactivated but since it is called only at the initial state I think that this would be an overkill.

[Maxwell-Rosen]

It sounds like there should be a separate 2x and 3x updater. Putting this if statement makes me think this function should be renamed. There is also an if statement for cdim==3, so this might be preferred to go inside there.

[manauref]

which regression test specifically?

i've been running rt_gk_sheath_2x2v_p1 without problems