fix opm-parser find rule

deals with the fallout of opm-parser #365 and #366
Merge pull request #236 from akva2/fix_opmparser

fix opm-parser find rule
[bugfix] use SparseLU when UMFPack was not found.
Merge pull request #238 from dr-robertk/PR/sparselu-when-no-umfpack

[bugfix] use SparseLU when UMFPack was not found.
PVT properties: allow them to be temperature dependent

Note that this patch does not introduce any real temperature
dependence but only changes the APIs for the viscosity and for the
density related methods. Note that I also don't like the fact that
this requires so many changes to so many files, but with the current
design of the property classes I cannot see a way to avoid this...