EclipseWriter: some fixes

- the output dir must not be empty or else file names like "/FOO.INIT"
  are generated which normally result in a segfault (unless you run
  this as root, which you _really_ should not)
- for SPE1, the output writer must also support specifying the grid
  via the D[XYZ]V keywords instead of relying on the SPECGRID keyword

with this, opm-autodiff's `sim_fibo_ad` does not crash anymore and produces usable restart files...

(these only contain the 'PRESSURE' keyword so far, though.)