Fix isw_calc_Er=2 MtOvR_spec bug, document Om_tE input, add roundtrip tests

[krystophny]

Summary

• Bug fix: isw_calc_Er=2 read Om_tE from the profile but never computed MtOvR_spec from it. The output path (isw_calc_Er >= 1) would try to write an unallocated array. Now write_multispec_output_a has an ELSE IF (isw_calc_Er .EQ. 2) branch that computes MtOvR_spec via Om_tE_to_MtOvR_spec.
• Doc-comment fix: isw_calc_Er in ntv_mod.f90 now describes all three modes (0/1/2) instead of just "on/off".
• Ambipolarity warning fix: The else-clause said "isw_calc_er is 0" even when the value was 2; now prints the actual value.
• Reference input: DOC/neo2.in.ql-full now documents isw_calc_Er=0/1/2 and the Om_tE namelist parameter.
• Tests: Fortran roundtrip (mode 1 output -> mode 2 input -> identical MtOvR_spec) and perturbation (half Om_tE -> half MtOvR). Python HDF5 roundtrip, absence, and half-profile tests.

Verification

Tests pass after fix

$ make test
Test project /home/ert/code/NEO-2/build/TEST
    Start 1: nrutil_test
1/2 Test #1: nrutil_test ......................   Passed    0.08 sec
    Start 2: er_rotation_test
2/2 Test #2: er_rotation_test .................   Passed    0.08 sec

100% tests passed, 0 tests failed out of 2

$ cd python/test && python -c "from test_generate_multispec_input import *; \
  test_omte_profile_hdf5_roundtrip(); test_omte_absent_when_not_provided(); \
  test_half_omte_differs(); print('All tests passed.')"
All tests passed.

Test plan

• Fortran unit tests pass (make test)
• Python multispec tests pass
• CI golden-record tests pass