Add example data to make possible additional unit tests

[aphalo]

The current version of the 'photobiology' package has extensive support for operations on multiple spectra stored in long-form _spct objects, but is still missing example data usable in unit tests for these operations. To some extent this also affects collections of spectra. The tests could be implemented using artificial data created on-the-fly but for the documentation, some examples would be also needed. We could assemble these from individual spectra at load time, but then they would not be a reliable reference for unit tests. For these tests to be exhaustive, the spectra should contain all the metadata attributes currently implemented.

• [x] Add example data for five different sunlight spectra as a source_mspct object
• [x] Add example data for five different sunlight spectra as a source_spct object
• [ ] Add example data for five different sensor response spectra as a response_mspct object
• [ ] Add example data for five different sensor response spectra as a response_spct object
• [x] Add example data for two different filter spectra as a filter_mspct object
• [x] Add example data for two different filter spectra as a filter_spct object
• [ ] Add example data for two different reflector spectra as a reflector_mspct object
• [ ] Add example data for two different reflector spectra as a reflector_spct object
• [ ] Add example data for two different object spectra as a object_mspct object
• [ ] Add example data for two different object spectra as a object_spct object
• [ ] Add example data for two different counts-per-second spectra as a cps_mspct object
• [ ] Add example data for two different counts-per-second spectra as a cps_spct object
• [ ] Add example data for two different raw-counts spectra as a raw_mspct object
• [ ] Add example data for two different raw-counts spectra as a raw_spct object
• [ ] Add example data for two different molar absorptance spectra as a solute_mspct object
• [ ] Add example data for two different molar absorptance spectra as a solute_spct object
• [ ] Add example data for two generic spectra as a generic_mspct object
• [ ] Add example data for two generic spectra as a generic_spct object

[aphalo]

Need to think how not to bloat the package, and add only the strictly needed data sets. Push forward to next milestone.