Also replace all calls to isLiquid by equivalent calls to isGas

[freeflow][rans] Set velocity gradient to zero, if it is a single element in on direction

See merge request !1130

[feature] test newtonconvergencewriter on 2pni test

See merge request !1139

[fluidstates] include necessary header in compositional.hh

See merge request !1138

[doc] Add preliminary contribution guide and style guide

See merge request !992

[multidomain][stokesdarcy] Improve docu and structure of coupling indices

See merge request !1126

* internally forward to isOnWallAtPos
* needed for coupling

[material][1padapter] Make usable for immiscible fluidsystems

See merge request !1129

Kilian Weishaupt authored Jul 24, 2018 and Timo Koch committed Jul 24, 2018

* choose correct number of compoments based on whether MultiPhaseFluidSystem
  is immiscible or not

* use helper struct instead of enable_if

[properties] improve the output of the type/property tag tree

See merge request !1120

[1p_1p] Ignore temporary grid files

See merge request !1121

[1pnc] Remove unused private variable

See merge request !1128

* porosity_ not used anywhere

Added PNG/dalton1.png and PNG/dalton2.png to the list of files for building the handbook.

See merge request !1127

Fix/remove phase idx

Closes #529

See merge request !1125

[richardsNC] Remove fluidsystemPhaseIdx

See merge request !1123

Free vtkoutputmodule from TypeTag

See merge request !1113