I brushed up on CMake file regarding fetching Boost.

Currently, saltatlas's CMake fetches Boost from the hard-coded URL.

This PR version still fetches a proper version of Boost automatically by default (thus, no changes are required in other projects that use saltatlas).
I added two CMake options to change the behavior as follows:

• BOOST_SOURCE_DIR
  • Path to Boost libraries directory already downloaded and uncompressed.
  • This option will copy the only required files to build this project from the original source directory. Useful for faster build and saving disk space.
• BOOST_FETCH_URL
  • URL or file path to an archived Boost source.

For both cases, Boost must be a version that supports CMake.
Using two options at the same time will result in an error.