[FEATURE] bspline support using eol bsplines implementation

This pull request adds a BSpline implementation based on the ell-eol-spline library. The library (a handful c and h files) is added as 3rd party lib to OpenMS (see below).  The library itself is header only, so no special care has to be taken w.r.t. building it. 

This pull requests actually introduces also a new concept to the build system to include smaller libraries in `openms/3rdparty` instead of adding them to the contrib. Here we have direct control over the build system inside the OpenMS build system. No need for a find_package(XX) and special care in the contrib. This mechanism should be  used for special purpose sub-libs like eol-bspline, numpress, or the ims stuff. The later two would have to be ported out of the OpenMS code. This also enforces the necessary separation between code we **use** and code we **develop**.

The bspline implementation is required to reintroduce the `TransformationModelBSpline` removed during the GSL cleanup. The only modification of the bspline code is contained in 8b0065765ad9238d86a64ee807a89e054b5c2a2c and fixes an issue with llvm/clang compilers (see http://clang.llvm.org/compatibility.html#dep_lookup_bases for details).

Also contains an unrelated commit fixing the mac os x install docu.
I did not write this part but I think because a forward declaration is then sufficient.