Make getIJK non-throwing and fix parallel globalCell

Previously we erroneously initialized a local variable in function CpGridDat
Therefore CpGridData:global_cell_ never contained any entries.
We should of course initialized the class member instead, which is done by this PR

As I noticed during the discussion with @andlaus in #87 the CpGrid::globalCe
contains meaningful global values even in the distributed case. Therefore
CpGrid::getIJK will return correct values and there is no reason to throw an
exception, which is removed by this PR.

This a substitute for #87 that fixes the real problem discovered during the discussion