FF = addnoisef(F) adds 10% normally-distributed white noise to the
vector/scalar field(s) F.
FF = addnoisef(F, EPS) specifies that the standard deviation of the
white noise is EPS times that of the field(s) F (EPS=0.1 by default).
FF = addnoisef(F, EPS, OPT) specifies whether the noise is additive
(OPT='add', by default) or multiplicative (OPT='mul').
FF = addnoisef(F, EPS, OPT, N) first filters the noise at scale N (in
mesh units) before adding/multiplying it to the field(s) F.