Implemented insert, pop, remove

In the set subroutine, the type of the data to store was integer, not generic.

Including a test.

Supports basic operations, but some more should be done.
No tests yet.

Some stacks and test cases for them.

Including a toy model test case.

Implemented in a generic way using the preprocessor,
with implementations for integers and reals.
Includes a little test case.

Added a README.