[MireilleSchneider]

TORAX converts its positive internal currents to the IMAS COCOS convention by flipping Ip and the current densities when writing
core_profiles, and the equilibrium writer flips B0. The core_profiles writer did not flip vacuum_toroidal_field.b0 nor global_quantities.v_loop, so for a standard ITER case (Ip < 0, B0 < 0) the output had b0 > 0 and v_loop with the sign of the internal current. This violates the COCOS 17 consistency rule sign(q) = sign(Ip * B0) checked by IMAS-Validator, and gives a loop voltage of the wrong sign relative to Ip (v_loop is an "ip_like" COCOS quantity, independent of B0).

• output/core_profiles.py: write -B_0 and -v_loop_lcfs.
• input/core_profiles.py: read v_loop_lcfs as -global_quantities.v_loop,
  consistent with the Ip flip already applied on input (B0 is already
  read as an absolute value).
• Add test_b0_and_v_loop_signs_follow_imas_cocos.