Unit tests with the "check" tool

Unit tests are needed to make sure the library follows certain specifications. It will also ensure that working functions keep working the same way when they are modified. The "check" tool for unit testing C programs has been chosen to implement these unit tests. To each C source file in the library, there is an equivalent unit test C source file with a named derived from the original source. It is therefore easy to find a particular test.
We should be able to measure the code coverage of the tests with the gcov tool that comes with the gcc compiler.