Increase coverage

[jlaehne]

We have some low hanging fruit in increasing the test coverage of LumiSpy:

• [x] px_to_nm_grating_solver in signals/luminescence_spectrum.py is not covered [See codecov, lines 291-310]

• [x] For to_eV and to_invcm in signals/luminescence_spectrum.py, the case where the variance is a single int/float is not covered when inplace=True. See codecov, lines 154-161 & 309-316

• [x] scale_by_exposure in signals/common_luminescence.py is missing coverage for the situation where ("Signal.quantity") == "Intensity (Counts)" See codecov, lines 154-155

• [x] remove_spikes in signals/cl_spectrum.py is missing coverage for show_diagnosis_histogram See codecov, line 92

• [x] solve_grating_equation in utils/axes.py is missing coverage for axis being non-uniform DataAxis See codecov, line 485

• remove_background_from_file misses some tests for warnings/errors, but as it will be deprecated is not worth implementing.

[jordiferrero]

I assign px_to_nm_grating_solver in signals/luminescence_spectrum.py is not covered [See codecov, lines 291-310] to myself. Will work on it in the next 2 weeks.

[jordiferrero]

Coverage 100%