water viscosity: calculate it the same way as documented by the Eclipse RM
FindERT.cmake: Look for libecl_well library
FindQuadmath: add -fext-numeric-literals if the compiler supports it

this is required for GCC >= 4.8 to support the 'Q' suffix for floating
point literals (which are used in the quadmath.h header)

In order to prevent compilers which eat the flag but do not support it
from complaining constantly, it is tested for with "-Werror" enabled...
add check-commits target
synchronize build system with opm-core

for some reason, parts of d1020930. must have been a screw-up of the
opm-material maintainer...
PiecewiseLinearTwoPhaseMaterialParams: use the same API as SplineTwoPhaseMaterialParams

this allows to use the two by changing the class name but without
requiring any further code changes...