Expand tilde used in command-line arguments

This enables one to specify e.g. --with-opm-core=~/opm/bld/opm-core
Use config mode if possible when finding packages
code is now included using \snippet. Apparently this looks better with the new Doxygen version. The HTML_EXTRA_STYLESHEET is now used rather then the HTML_STYLESHEET in order to include used-defined styles for the same reason
Handle ERT not found condition gracefully

If the files for ERT is not found, then the find module cheats and
indicates that it didn't compile either (since there are some later
tests for HAVE_ERT).

However, erraneously, it gave this variable the value 0, which tests
for false in the CMake code but not in the #ifdef checks in the C++
code. This error is cancelled in opm-core by the fact that it didn't
store it as a cache variable, but resurfaced in opm-polymer when it
probes the second time (as both opm-core as a dependency and
opm-polymer itself declare a dependency on ERT).

It is now stored in a way that should be satisfactory to both
projects.
Merge pull request #172 from rolk/172_confmode

Encourage the use of config mode if available
Merge pull request #173 from totto82/doxygen

changes to tutorials and doxylocal
Merge pull request #174 from rolk/173_havecached

Handle ERT not found condition gracefully