changed to using input type unit convertions for field props

[hnil]

-- make input and output consistent without more than input definition -- simplifies output of field props with combined units (i.e. length^i time^j temperature^k

[hnil]

need checking in opm-simulators before merging.

[bska]

One more comment. We already have a UnitSystem object in InitIO::write() and that one is supposed to be the same as—or at least equivalent to—the one stored in the FieldProps object. If we look narrowly at the task of writing the output file, we might not need the new member functions in FieldProps or FieldPropsManager, but I am not opposed to adding them either.

[hnil]

Maybe we should have one UnitSystem at least of each type. And deside for that. The one I points to is the one used for in put I believe.

[bska]

The one I points to is the one used for in put I believe.

That is true, but so is the one in InitIO::write().