fix bug for PercentageValue

[zfb132]

I am using the meshlab GUI software to generate the pymeshlab filter script. However, I got an error when pasting the code to a Python file. For example:

>>> ms.generate_sampling_clustered_vertex(threshold = pymeshlab.PercentageValue(1.100005))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'pymeshlab' has no attribute 'PercentageValue'

This PR should fix the bug above.

[alemuntoni]

Actually, PercentageValue is correct. devel branch of MeshLab is aligned with devel branch of PyMeshLab, and both are still in development. And in the devel branch of PyMeshLab, the class is called PercentageValue. It has been renamed for consistency with other class names.