Improve 'make_vars' and 'internal' methods

[Pierre Denis]

The make_vars method should be improved so to avoid passing a dictionary at each call. A dictionary like globals() should be passed once for all by a new dedicated function. The make_vars could even be removed, enhancing the as_joint method.

After this is done, the internal method could be upgraded to display variable names, upon request.

[Pierre Denis]

Remove Lea.make_vars method, add Lea.declare_namespace method, add create_vars argument on some methods (refs #82)

→ <<cset ce9f44da9702>>

[Pierre Denis]

Done in Lea 4

[Pierre Denis]

Put create_vars argument on 'read_bif_file' method (refs #82)

→ <<cset ad95ed2ebfc2>>

[Pierre Denis]

Update UT of 'read_bif_file' method (refs #82)

→ <<cset 3ffc30a59dca>>