Add the warn_continuum_below_threshold checker to all the analysis functions where it's relevant

As per the plan in https://github.com/astropy/specutils/issues/535#issuecomment-541783378,
after #546 has been done, *if* there's not big performance penalties, the decorator implemented in #538 should be expanded to all the analysis functions that require continuum-subtracted input spectra.

Read the plan in issue #535, especially comment 541783378, then review the implementation from issue #538 and the completed work in #546. Identify the analysis functions that require continuum-subtracted input spectra and assess the performance impact. Done means the checker is applied to all relevant functions without a significant performance penalty.

Written by the indexing model from the issue text.