Make Mono.d_spacing conditionally required per XDI/1.0 spec

[smrgeoinfo]

Summary

The XDI/1.0 specification requires Mono.d_spacing only when the abscissa (Column.1) is monochromator angle or encoder step count — the mono d-spacing is needed only to convert those units into energy:

The Mono.d_spacing header field must be specified if the abscissa is conveyed as monochromator angle.

The current schema requires Mono.d_spacing unconditionally, which produces spurious errors on the many XDI files whose abscissa is energy or wavelength (where d_spacing is optional).

Fix

• Remove the unconditional "required": ["d_spacing"] from the mono subschema.
• Add a top-level allOf conditional that requires mono.d_spacing only when column.1 matches ^(angle|encoder)(\s|$).

Tests

• test_missing_mono (whose fixture has Column.1: energy) is replaced with test_mono_dspacing_not_required_for_energy_abscissa to reflect the corrected semantics.
• test_WriteToFile / test_WriteToFileTwo: invalid_data setup now also switches the abscissa to angle deg so removing d_spacing still triggers the mono error path.
• New tests/test_MonoDspacingConditional.py covers three cases:
  • angle abscissa, no d_spacing → mono error
  • angle abscissa, with d_spacing → no mono error
  • energy abscissa, no d_spacing → no mono error
• New fixtures: tests/missing_dspacing_angle.xdi, tests/valid_angle_dspacing.xdi.

Test plan

• python -m pytest tests/ — 26/27 pass on this branch. The one remaining failure (test_WrongFields::test_invalid_column) is pre-existing on main (unrelated to this change).
• Manually confirmed with a real-world energy-abscissa file that previously errored on mono.d_spacing: after the fix, no mono error is raised; when the same file's abscissa is edited to angle deg, the mono error correctly fires.

Context

Encountered while integrating xdi_validator into a CDIF-XAS metadata pipeline against a corpus of ~100 real-world XDI files from Diamond Light Source beamline B18 (all energy abscissa). Every file was being flagged as invalid solely on the unconditional Mono.d_spacing requirement, which the XDI/1.0 spec does not actually mandate for energy abscissae.

[AAAlvesJr]

Dear @smrgeoinfo, thank you very much for this important contribution. Just reviewed your PR and everything seems fine for me. I will merge it now and prepare a new release to make the changes promptly available for the other users.