#119: accept suite.rc interval configuration in ISO 8601 formats

This changes the syntax for specifying intervals of time in the
`suite.rc` file to use ISO 8601 interval syntax.

@hjoliver, please review.
I generally just take Rose team code styling on the basis that you guys have at least discussed style and digested the official Python style document (and I, embarassingly!, still haven't), but I'm curious about this change. To my eye, while the original is a bit "long of line" it is considerably easier to read, it doesn't use an unnecessary temporary variable (```delay```), and it uses the config items more consistently - could you explain a bit?