User report: reading values from grid with lgr

Users have reported that reading data from runs with lgr seems unintuitive. They were interested in finding the values from the _main_ grid in the case with lgr included. Although the _main_ grid is not used for simulation, averaged values are still stored for book keeping, and it is therefore possible to extract comparable values in both scenarios.

Approach:
```
# Simulation case without lgr

original_grid = ecl.EclGrid('ORIGINAL.DATA')
original_rest = ecl.EclFile('ORIGINAL.UNRST')
original_init = ecl.EclFile('ORIGINAL.INIT')

# Simulation case with lgr

lgr_grid = ecl.EclGrid('LGR.DATA')
lgr_rest = ecl.EclFile('LGR.UNRST')
lgr_init = ecl.EclFile('LGR.INIT')

tstep = 10

x = 1
y = 1

# Read saturation for both runs

original_soil = original_rest.iget_named_kw("SOIL", tstep)
lgr_soil = lgr_rest.iget_named_kw("SOIL", tstep)

assert (original_soil != lgr_soil)

lgr_soil = lgr_rest.iget_named_kw("SOIL", tstep * 2)

assert (original_soil == lgr_soil)
```

They found that length of `lgr_soil` is twice the length of `original_soil` (likely n-times the length, where n is number of lgr's + 1). What is a bit curious though is that the values where tstep is the same, didn't make much sense. They did not compare correctly with the values in the `lgr` grid. Still, reading from cells in the `lgr` grid, from what I can recollect, should be performed by fetching the `lgr` and then retrieving values from that. I have not looked any more into this.

Hopefully easy to reproduce.