Make fluidsystems more generic and restructure

Fluidsystems could be restructured in the following way + made more generic:

• Fluidsystems/immiscible
  • 1pgas, 1pliquid (former gasphase.hh, liquidphase.hh, probably only need to be renamed. This is related to issue #403 (closed))
  • 2pliquidgas, 2pliquidliquid (need to be created)
  • 3pliquidliquidgas (needs to be created)
• Fluidsystems/miscible
  • 2p1c (inherits from 2pliquidgas only with the same two components)
  • all other fluidsystems

Add isMiscible to all fluidsystems. It might be possible to delete some specific fluidsystems and use the new generic fluidsystems instead.