opalimpact-1 failing after replacing OFFSETZ with ZSTART

Since we discussed to get the regression tests working, I had a more detailed look at one of them.

After replacing OFFSETZ with ZSTART in commit e2e902d8 the opalimpact-1 test is failing.

I believe this is because the starting s position is different in both cases, perhaps contrary to what one would expect(?).

In the reference .stat file there is (t,s):

1.000000000000000e+00         	1.383007003378306e-02
2.000000000000001e+00         	2.766014006756613e-02

So the reference starts at t=0.0, s=0.0

and the current version (t,z):

2.115406793793548e+00         	2.883007003378255e-02
3.115406793793549e+00         	4.266014006756509e-02

The current version starts at t=0.11 , s = 0.15 (value of ZSTART).

since the test has ZSTOP = 1.0 the last line will be different and the test will fail.

One possible way to fix the test is to have ZSTOP=1.15.

I haven't checked the other tests that were changed in the same way, but these might also be affected in the same way.