[fluidsystems] remove isLiquid in favor of isGas

...fluidmatrixinteractions/2p/thermalconductivitysomerton.hh

dumux/material/fluidsystems/1padapter.hh

dumux/material/fluidsystems/1pgas.hh

dumux/material/fluidsystems/1pliquid.hh

dumux/material/fluidsystems/2p1c.hh

dumux/material/fluidsystems/2pimmiscible.hh

dumux/material/fluidsystems/3pimmiscible.hh

dumux/material/fluidsystems/brine.hh

dumux/material/fluidsystems/brineair.hh

dumux/material/fluidsystems/brineco2.hh

dumux/material/fluidsystems/h2oair.hh

dumux/material/fluidsystems/h2oairmesitylene.hh

dumux/material/fluidsystems/h2oairxylene.hh

dumux/material/fluidsystems/h2oheavyoil.hh

dumux/material/fluidsystems/h2on2.hh

dumux/material/fluidsystems/h2on2o2.hh

dumux/material/fluidsystems/liquidphase2c.hh

dumux/material/fluidsystems/spe5.hh

dumux/porousmediumflow/2p2c/volumevariables.hh

test/material/fluidsystems/checkfluidsystem.hh