[Opm] New dependency

[Joakim Hove]

I have just added the C++ formatting library fmt as a dependency to 
opm-common. See discussion here: https://github.com/OPM/opm-common/pull/1930

Binary packages exist for Ununtu, Debian and RH (epel).

There are no users of the fmtlib added to the code yet - so if this 
turns out to be problematic we will revert it.

Joakim

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