possibility to extend elements in parser is broken

[Jochem Snuverink]

according to the old manual, this was possible:

d1: drift, l=0.5*m, phi=0.1;

! extend/ alter properties
d1: aper=0.1*m, l=0.2*m;

But this is no longer working. I think it is useful to have and will fix it.

[Jochem Snuverink]

fixed with commit 6788002