Add convenience Makefile
The Makefile is just a convenience to more easily build elsa. To build all of elsa, just call `make build` from the top level directory. You can also pass any target to build, to specifically build this target, e.g. `make build test_DataContainer`. Other useful commands could `make tests`, to build and run all tests. `make test <test-name>` to build a specific test case and run it. And `make watch <test-name>` to build and run a test continuously on code changes in elsa. Note that a sub-folder structure will be created of the form `build/$BUILD_TYPE/$COMPILER`. This enables quick iteration for different build types and compilers, via passing the options `BUILD_TYPE` and `CXX`.