define a Workplane.export() (#1153)

[aavogt]

I tried to put it in cq.py under class Workplane, but there were problems with circular imports.

The docstring is not quite right since :w: should probably be :self:.

cq.Workplane.export.__doc__
'\n    Export Workplane or Shape to file. Multiple entities are converted to compound.\n\n    :param w:  Shape or Workplane to be exported.\n    :param fname: output filename.\n    :param exportType: the exportFormat to use. If None will be inferred from the extension. Default: None.\n    :param tolerance: the deflection tolerance, in model units. Default 0.1.\n    :param angularTolerance: the angular tolerance, in radians. Default 0.1.\n    :param opt: additional options passed to the specific exporter. Default None.\n ```