Changes in visitor branch:
o import_dxf(): layername="" imports all layers. Importing a single layer with a zero-length name is no longer supported. FIXME: The same prob. goes for dims
o cylinder(): the r parameter will now always be used in place of a missing r1 or r2
o for():
  - with scalar argument now works, e.g.: for (i=23) echo(i)
  - empty for loop is not evaluated, e.g. for () echo(i) 
  - for loop with illegal value is not evaluated, e.g. for ([0:true:2])
