[Opm] Dependencies revisited

[Joakim Hove]

Another recurring issue is Boost. If we jump to Debian Buster and Ubuntu
> 19.04, this entails that we can bump minimum version of GCC to 8.3.0. Is
> this sufficient to consider removing Boost as a dependency?
>

The parser uses boost spirit::qi which at some stage proved to give a
performance gain for plain parsing of numerical input, i.e. "3.14" -> 3.14.
It is simple to replace with std varieties, but there might be a
performance hit.

J