ERT-743: EclSUm.get_interp() works for days=0

Conflicts:
	devel/python/python/ert/ecl/ecl_sum.py
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.
Changed version number to 1.7.5