ERT-743: EclSUm.get_interp() works for days=0
ERT-743: Special case for interpolation at start

In general the time intervals are halfopen to the upper end when mapping
from true sim_time to a ministep block, but there is a special case when
interpolating at the the start of the simulation.
Merge pull request #595 from joakim-hove/ERT-743-eclsum-interp

Ert 743 eclsum interp