Merge branch 'feature/various-small-fixes' into 'master'

dumux/geomechanics/el2p/el2pamgbackend.hh

dumux/geomechanics/el2p/el2passembler.hh

dumux/implicit/nonisothermal/nivolumevariables.hh

dumux/material/binarycoefficients/h2o_o2.hh

dumux/material/binarycoefficients/n2_o2.hh

dumux/material/fluidsystems/brineairfluidsystem.hh

test/common/generalproblem/test_generalproblem2p.cc

test/common/propertysystem/test_propertysystem.cc

test/common/spline/test_spline.cc

test/freeflow/navierstokes/test_navierstokes.cc

test/freeflow/stokes/test_stokes.cc

test/freeflow/stokes2c/test_stokes2c.cc

test/freeflow/stokes2cni/test_stokes2cni.cc

test/freeflow/zeroeq/test_zeroeq.cc

test/freeflow/zeroeq/test_zeroeq_channel.cc

test/freeflow/zeroeq2c/test_zeroeq2c.cc

test/freeflow/zeroeq2cni/test_zeroeq2cni.cc

test/geomechanics/el1p2c/test_el1p2c.cc

test/geomechanics/el2p/test_el2p.cc

test/geomechanics/elastic/test_elastic.cc