Write the cos part of grpmn and bvec for lasym

[CharlesCNorton]

grpmn and bvec carry a second symmetry index for lasym, ndim = 2, but the debug output writes only the first slice. analyt.f90 says so at the call: ! missing dim: (ndim: 1 or 2). The cos half is therefore not reachable from the dumps, and a consumer comparing against them can only check the sin half of an asymmetric vacuum solve.

This writes it as grpmn_cos and bvec_cos at the four sites that dump either array, under the existing lasym guard, and adds the matching add_null("grpmn_cos") where grpmn is already nulled for ivacskip /= 0. The existing keys are untouched, so anything reading the current output is unaffected.

Checked on a free-boundary lasym = T cth-like case at ns = 15, mpol = 5, ntor = 4, nfp = 5: vac1n_analyt, vac1n_fourp and vac1n_fouri each gain the cos key, and VMEC++ reproduces both halves to 1e-9.