Reformat prerequisite file according to convention

This commit makes a few adjustments to the white-space of file
'opm-parser-prereqs.cmake' to honour the conventions of the other
*-prereqs.cmake files within the OPM project's module suites.

No functional changes.
Depend on Boost.Iostreams

The benchmark library uses Boost::iostreams to do decompression. Since
we only scan for the Boost dependency once, this submodule is added to
all of the projects in order to have a coherent dependency on Boost.
Merge pull request #134 from bska/uniform-parser-prereqs

Uniform parser 'prereqs' file