Provide ResultsList.get_step_where: closest index to a given time
Provide ResultsList.get_step_where: closest index to a given time

Closes #1713 by finding the index closet to a user-requested
point in time. Support for various units is provided through
ResultsList.get_times (s, d, min, h) and there is support for
absolute and relative tolerances.

mit-crpg/openmc,walshjon/openmc,amandalund/openmc,paulromano/openmc,paulromano/openmc,shikhar413/openmc,walshjon/openmc,walshjon/openmc,shikhar413/openmc,walshjon/openmc,amandalund/openmc,paulromano/openmc,shikhar413/openmc,amandalund/openmc,amandalund/openmc,shikhar413/openmc,mit-crpg/openmc,mit-crpg/openmc,paulromano/openmc,mit-crpg/openmc