Add Init to Spectrum Widget Folder

[JackEAllen]

Issue

Closes #2442

Description

MI fails to start with the error message
ModuleNotFoundError: No module named 'mantidimaging.gui.widgets.spectrum_widgets'

Looks like an import error, where something is working fine from the source directory but not in the package.

After further investigation, A missing __init__ seemed to be the culprit which has now been added to mantidimaging/gui/widgets/spectrum_widgets/

Testing

Describe the tests that were used to verify your changes.

• Build locally with make build-conda-package and install into mantidimaging-nightly using mamba install /home/sam/mambaforge/envs/build-env/conda-bld/linux-64/mantidimaging-2.9.0a1.post402-py312_1.tar.bz2
  • Verify if that that package bultds correctly and can be installed and ran wihtout any import errors

Acceptance Criteria

Mi can build successfully without returning a ModuleNotFoundError error

Documentation

How have you changed the documentation to reflect your changes? All changes should be noted in the appropriate file in docs/release_notes

NA - This is a flash PR and does not change MI in any significant way.

[coveralls]

coverage: 73.618% (+0.002%) from 73.616%
when pulling 77002df on 2442-spectrum_import_on_build_error
into 2da9f72 on main.

[samtygier-stfc]

If I build locally with make build-conda-package and install into mantidimaging-nightly with mamba install /home/sam/mambaforge/envs/build-env/conda-bld/linux-64/mantidimaging-2.9.0a1.post402-py312_1.tar.bz2, it fixes the issue.

Doing the same with main I get the ModuleNotFoundError.