This means that we can just rely on coolprop to pass on its dependencies,
instead of tracking possible changes in them.
The submodule is right now checked out to the latest stable version of
coolprop, namely 6.4.3.

This way, the failure to implement needed methods on base solver
is caught at compile time, instead of runtime.
Also moved the docs in the header file.

These do have default implementations.

In all methods that are not actually implemented, an error is reported.

…ld and ld.

The MSVC linker should include only explicitly exported symbols, so
it doesn't need this (not tested!). All other linkers are not really supported here,
unless they behave like the MSVC linker.

Also: Make the ModelicaUtilisties library that is included, so the test
can be built a shared library.

1. The Sources directory was replaced by a directory ExternalMediaLib.

2. The sources of ExternalMediaLib are now grouped into differenct static
libraries with clear interdependencies, such that adding a new external solver
should be straight-forward: To do this, you need to

    a)add a new directory Solver<Name_of_your_external_library> inside
    Projects/ExternalMediaLib

    b) copy the CMakeLists.txt from SolverCoolProp and
    change it to your needs

    c) Define a new cmake variable (like COOLPROP) and add building logic to
    Projects/CMakeLists.txt

    d) Conditionally on this variable include and link
    Solver<Name_of_your_external_library> inside
    Projects/ExternalMediaLib/CMakeLists.txt

    e) Conditionally on this variable include your header file (best located in
    Solver<Name_of_your_external_library>/include in solvermap.cpp and add
    corresponding code to instantiate your solver in solvermap.cpp

3. In addition CoolProp is now a static library, so that dependencies to it can
be managed with CMake, instead of manually in the ExternalMediaLib
CMakeLists.txt.

4. Note that the FluidProp interface is untested, because I have no license for it.

5. The symbols exported from externalmedialib.so when compiling and linking with
clang or gcc are now only those also exported by msvc, so that it is safe to
link to another instance of the coolprop library and in general have symbols
defined that are named as those in externalmedialib.

6. The next commit will introduce a check for inter process optimization (a.k.a.
link time optimization) to remove the possible runtime penalties introduced by
the sub-division into static libraries.

…lation

This of course reduces performance and should not be used for building
binaries for production usage.

also: explained the existence of the targets modelica_static_helper and
modelica_utilities_DONT_USE

in their CMakeLists.txt files.

(different function signatures between declaration and implementation.