Feature/free boundary condition

The aim of this kind of boundary condition descriptor is to indicate
that as "special" treatment is to be performed for a boundary that
is not subject to a boundary condition at the continuous level.

For instance it may occur when using a centered scheme to discretize a
system which has only one wave (quite often with scalar laws:
transport, traffic flow,...)

It can be used both to change locally the scheme and to check that no
boundary has been forgotten in the description