black oil fluid system: rename surfaceDensity() to referenceDensity()

because it seems like what eclipse uses in the DENSITY keyword is not
really for surface pressure but at the pressure where the phase's
formation volume factor is one. (this pressure does not seem to be
specified and can potentially be different for every fluid phase.)
Thus rename surfaceDensity() to referenceDensity() and remove the
logic to allow volume formation factors which are not equal to one at
the reference pressure. (i.e., formerly the surface pressure.)
black oil fluid system: fix stupid bug when setting the gas viscosity

gas viscosity != gas formation volume factor (even if the values are
somewhat similar)
make the regex test more thorough

it seems like the std::regex delivered up to gcc 4.8 has problems with
constructs such as ".+". (at least if they appear at the end of a
regex...)
build system: handle the threads library in the ERT module exactly as opm-core