new method emr_cut

[nettam]

Add a function similar to emr_dist that cuts the iterator patient/time by expr and breaks.
API:

emr_cut
function (..., include.lowest = FALSE, right = TRUE, stime = NULL, 
    etime = NULL, iterator = NULL, keepref = FALSE, filter = NULL, 
    names = NULL) 

...: pairs of [expr, breaks], where ‘expr’ is the track expression
          and breaks are the breaks that determine the bin or 'NULL'.

include.lowest: if 'TRUE', the lowest (or highest, for 'right = FALSE')
          value of the range determined by breaks is included

   right: if 'TRUE' the intervals are closed on the right (and open on
          the left), otherwise vice versa.

   stime: start time scope

   etime: end time scope

iterator: track expression iterator. If 'NULL' iterator is determined
          implicitly based on track expressions. See also 'iterator'
          section.

 keepref: If 'TRUE' references are preserved in the iterator.

  filter: Iterator filter.

   names: names for track expressions in the returned dataframe