[fluidsystems] Add isMiscible() method

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/base.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/h2oheavyoilfluidsystem.hh

dumux/material/fluidsystems/h2on2.hh

dumux/material/fluidsystems/liquidphase2c.hh

dumux/material/fluidsystems/spe5.hh

dumux/material/fluidsystems/steamn2cao2h2.hh