Automatically detecting R peak detection failure

[e-pet]

As outlined in the Readme, most algorithms provided in the toolbox rely on (at least reasonably) accurate R peak detection. For this reason, it would be nice to have a means of automatically detecting when R peak detection fails. While some checks are already implemented (e.g. the min/max R peak intervals are reported on the command line, which should already give a good indicator), a more comprehensive assessment and an automatically generated warning/info message could be useful.