[Build System] Introduce Boost.Test as an Optional Prerequisite

[bska]

I'm about to do some work on vertical scaling of saturation functions, especially the features that are introduced by the ECLIPSE keywords KRWR, KRGR, KRORW, and KRORG. As part of this work I'd like to expand the set of unit tests in opm-material to ensure I don't make too many mistakes. The current set of unit tests don't use a testing framework however and instead rely on manually implementing function main().

Is there a hard limit on using testing frameworks in opm-material or may I write my new unit tests in terms of the Boost.Test library?

[akva2]

this is only for historic reasons and the reason is no longer around. go ahead.

[alfbr]

This can be an opportunity to move away from Boost.Test though. @joakim-hove , what do you think?