[patches] Update istl patch to 2.6

See merge request !1454

Feature/handbool spellchecl

See merge request !1452

[handbook][gnuplot] Improve wording, formatting

See merge request !1450

[doc][handbook] Add corner-point grid description

See merge request !1444

Fix/handbook developing

See merge request !1446

Fix/facet 1p convtest parallel

Closes #638

See merge request !1445

Fix/handbook numerics

See merge request !1442

When running the tests in parallel the box- and tpfa-specific tests were
using and overwriting the same grid file, which lead to invalid grids
during the convergence tests.

[doc][handbook] build directory name is not specified in input file

See merge request !1439

[doc][handbook] do not mention debug opts

See merge request !1433

The resolution of the png was very poor

debug.opts and optim.opts do not exist anymore. This commit adapts the
respective section in the handbook.

[cmake] Delete find package opm-common

See merge request !1436

Bugfix/turbulentdissipation nc

See merge request !1421

Opm-common changes some of the cmake variables
that are set by dune, e.g. HAVE_UMFPACK.
This should be fixed by these changes.

Cleanup/doxygen for 3.0 porousmediumflow

Closes #596

See merge request !1416

[test][facet][1pconvergence] remove UMFPack cmake guard

See merge request !1432

[test][facet][1pconvergence] do not use umfpack

See merge request !1429