<!--- Provide a general summary of the issue in the Title above --> <!--- Please label the issue with one of the following: Documentation, Outreach or Question --> ## Summary <!--- Describe the motivation behind the issue --> Currently there is no unit test coverage for the following functions - `cardiac.heart_rate` - `cardiac.heart_rate_variability` - `cardiac.heart_beat_interval` - `multimodal.retroicor` ## Next Steps <!--- Provide possible steps to take in order to address the issue -->