OpmFind.cmake: prevent multiple calls to the same module in the same run

this prevents to check for the same module more than once in the same
cmake run and should thus speed things up a bit as well as make the
output a bit cleaner. For this I assumed that the ${name}_FOUND cmake
variable does not get cached, which it seems like it does not...
cmake: make sure all calls to message() exhibit a type
OpmFind.cmake: also check for CAPITALIZED found variables

thanks to @rolk for catching this
Added pvt functionality for wetgas
The pvt interface is extended to handle wet-gas systems:
1. rvSat is added as a function in the PVT interface
2. SinglePvtLiveGas computes the pvt values and its derivatives
3. The old rbub variable is changed to rsSat for clearity
4. The new interface is tested in test_blackoilfluid with data from
liveoil.DATA and wetgas.DATA
Initialization using the RV keyword
The initial Rv value is specified using the RV keyword in the Parser.
The Rv values are used in the computation of the initial surface
volumes.
Change the extrapolation in the livegas pvts
The special extrapolation for pressure values above
the maximum table value is removed to sync with Mrst
Phase pressure is used for surface volumes calculations
Document modules and structures in the build system

Give a brief description of each of the modules that comprises the build
system, and the suffices that is used to form a virtual structure of
variables for each project.
Terminology instead of Notation as section header

Notation implies that we are going to be given symbols which
represents values or such (which arguably could be true since the
path is a particular value, and from programming we are used to
multi-letter symbols perhaps also with whitespace, but it's a
stretch), but Terminology is more accurate, as this section
describes fits the definition better: it is a vocabulary of
technical terms.

Hat tip: @bska
Merge pull request #456 from rolk/456_blddoc

Document modules and structures in the build system
Portability fix: 'uint' is nonstandard.
Merge pull request #457 from atgeirr/fix-uint

Portability fix: 'uint' is nonstandard.
fix some openmp pragma's
Merge pull request #459 from akva2/fix_openmp_pragmas

fix some openmp pragma's
Merge pull request #450 from andlaus/add_message_type

cmake: make sure all calls to message() exhibit a type
Fixing comments
The comment now currently say 1/Bg
Merge pull request #462 from totto82/wetgas

Added pvt functionality for wetgas
Merge pull request #448 from andlaus/prevent_multiple_package_detections

OpmFind.cmake: prevent multiple calls to the same module in the same run
Create aliases for probed variables

Headers from other libraries (notably DUNE) require definitions of
other variables than our probe process set (e.g. HAVE_UMFPACK instead
of HAVE_SUITESPARSE_UMFPACK_H). This module sets those aliases for
compatibility.
Declare a need to know if UMFPACK is available

Note that the variable HAVE_UMFPACK is not set by FindSuiteSparse,
but by OpmAliases.