Infer signal length when missing, add tff reader, allow setting remot…

…e url for downloading, add channel_names option for reading

Add datetime support for records, allow multi_to_single digital fmt i…

…n certain cases, large internal refactor, add force_channels argument to rdsamp

Fixes for comparisons, wfdb plots, and downloading databases

small module doc change

Multiple API changes to core objects and io functions. Add qrs detector

and evaluator.

fix various header regex, rename some processing variables, fix annot…

…ation plotting

implement inplace conversion for dac to save memory, and fix rdann sa…

…mpto case reading in one extra sample

fix dac over/underflow error

clean code

Refactor annotations library and change annotation attribute names