Add examples to all functions

[japhir]

Right now the basic documentation is almost there, but probably it's better to have a few example calls for each function.

• abundance_ratios()
• acid_fractionation()
• add_info()
• append_expected_values()
• append_ref_deltas()
• apply_etf()
• bulk_and_clumping_deltas()
• calculate_etf()
• clean_did_info()
• clumpedr_get_default_parameters()
• collapse_cycles()
• correct_backgrounds()
• delta_values()
• empirical_transfer_function()
• find_bad_cycles()
• find_init_outliers()
• find_internal_sd_outlier()
• find_outliers()
• find_param49_outliers()
• find_session_id1_outlier()
• find_session_outlier()
• get_inits()
• get_ref_delta()
• isobar_ratios()
• little_deltas()
• match_intensities()
• nest_cycle_data()
• parse_info()
• remove_outliers()
• revcal()
• spread_intensities()
• spread_match()
• summarise_outlier()
• summarize_outlier()
• tempcal()
• temperature_axis()
• temperature_calculation()

[japhir]

I've now added a lot of simple tests for each of the functions, maybe those can be used as a template for making simple example function calls?

See the https://github.com/isoverse/clumpedr/tree/master/tests/testthat directory for inspiration

[japhir]

I've added a list of all the functions to the first comment to keep track.