global Warnings flag

[demisjohn]

Migrate all functions to use global Warnings=True/False.

Implemented a WARN() function, with set_WARN() & unset_WARN().

To do this, make sure all func's that have a warn argument set it to False by default (global WARN() is True by default).

Then change
if warn: --> if warn or WARN():

and alter docstring to:

warn : { True | False }, optional
        Print a warning if the node name exists?  Defaults to False, but still prints if the global pyFIMM.set_WARN() is True, which it is by default.   Use set_WARN()/unset_WARN() to alter.

[demisjohn]

Implemented only in __Classes.py (Project object, check_node_name etc.) in a10dfdc