Add basic build support

[tanaya-mankad]

This branch adds cmake build scripts so that the lattice-generated C++ code can be tested for correctness. A few items still rest on the user, as spelled out in the additions to README.md.
Lattice will now initialize a C++ source directory as a git repository, and download necessary submodules. Building the C++ project is not yet part of testing, because it still doesn't support the schema's "forward declaration" syntax.

[spahrenk]

• #include <template.h>' and std::unique_ptr heat_source;now appear as#include <missing-type.h>andstd::unique_ptr heat_source;`
• Definition of initialize function for derived classes is no longer needed.
• Statements#include <ashrae205.h> and #include <core.h> missing in some files.