Change ASTNodeDataType construction and handling of type descriptors

From the ast point of view this gives cleaner data type description.
Nodes used to describe type names are now explicitly described as
type names. Previously,

- the data type could be marked as the described type. Now it is
referred as the `typename` of the type. For instance the `R^2`
type descriptor node was given `R^2` as a data type. Now it is given
the `typename(R^2)`: it is a typename which refers to `R^2`.
- In some other cases, it could have been just `typename` which forced
to re-parse entries (which is never good).

Also `affectations` types is fixed: it is now given the `void` type
instead of the type of the affected data.

Functions input and output spaces types should now be correctly
defined: some more tests are however required.

The case of compound types is however not that clear since the data
type of the type `R*R*R^2` is just defined as `typename(list)`. This
may be improved (in order to help checking of function definition/use.

An initialization issue related to tuples of R^1 was fixed on the way
and some error messages improved.

This is related to issue #21