Add a method to check if a signal with the given name exists.

Correct the name of the method isPlugged.

The old one is kept, but with a warning message
(for now).

[skip ci]

Fix MANDIR problem.

Pointed out by A. Mallet.

IVIGIT, added signal-helper (macros to ease the declaration of signals) and entity helper (macros to make the macros of signal-helper possible).

IVIGIT, added signal-helper (macros to ease the declaration of signals) and entity helper (macros to make the macros of signal-helper possible).

This crashes the linkage because of the particular link
with the frameworks (here the framework Acceleration).