moving shallow_water solver from thests to the library tree; libmpdata++-config.cmake

Some comments:

These changes was triggered by the need to package in a sensible way the elliptic-drop code at http://github.com/igfuw/shallow-water-elliptic-drop/

The reason to move shallow_water from tests into the library tree is to have it installed with "make install". This way any software that uses libmpdata++ can use the shallow-water solver not needing to ship a copy.

The Cmake "-config.cmake" file is now aalso installed into a location that enables external software to simply use "find_package(libmpdata++)" in their CMakeFile.

What remains to be done is to use find_package(libmpdata++) within libmpdata tests itself - now the tests/CMakeLists.txt file is almost a duplicate of libmpdata++-config.cmake.