Suboptimal folding

Introduced hook into subopt. This function can find suboptimal folds in a free
energy value neighbourhood of the minimum free energy.

It requires the definition of an array of structs (SOLUTION*) for obtaining the
results.
Memory deallocation

Introduced a mechanism for memory deallocation. Particularly subopt can return
a large number of results (occupying a big portion of memory) so all c
allocated memory must be freed.

When returning a c_char_p, ctypes automatically converts it to a string. This
prevents the possibility of freeing it. An alternative is to return a c_void_p
and casting it to a string when needed, keeping around the c_void_p for
deallocation.
Repackage as package

Introduced init and version files to allow library to work as a package with a
clean interface (the namespace is not polluted by ctypes and os).