Allows resetting maxh to larger values again:
```
from netgen.occ import *
from ngsolve import *
b1 = Box((-1,-1,-1), (1,1,1))
b1.faces.Max(X).maxh = 0.1
b1.faces.Max(X).maxh = 0.2
geo = OCCGeometry(b1)
mesh = Mesh(geo.GenerateMesh(maxh=0.5))
Draw(mesh)
```
Needed for example in meshing app. Before it was not possible to set
maxh to larger value again.
f808a2bb64