adding a draft of libmpdata++-config.cmake to enable CMake users to use find_package(libmpdata++)
adding a todo list to libmpdata++-config.cmake
moving shallow_water.hpp to libmpdata++/solver so that library users using shallow_water equations do not have to ship their own .hpp file
fixing path in tests/sandbox/spreading_drop_2d/spreading_drop_2d.cpp
fixing tests/unit/shallow_water
Merge pull request #157 from slayoo/master

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