Created a basic region of interest (ROI) object that accepts times of interest and string descriptors

Created a basic ROI class. It's current function is to deal with ROIs in time, for example an instrument rotation or SAA occurrence. Design as follows:

  - accepts either 1 or 2 times that can be understood by parse_time
  - these times are assumed to be start and end times respectively (if only 1 time is given, start and end times are identical)
  - accepts a string descriptor for the region of interest
  - accepts a source descriptor explaining the origin of the ROI (e.g. 'RHESSI', 'LYRA LYTAF')

Relates to issue #164