Create a Geopoints constructor that takes a filename as argument

[iainrussell]

Currently, users cannot call a constructor to create a new Geopoints variable - they have to be created through function calls.

We should expose this class more fully by offering a constructor that takes a file path as input argument and calls the mv.read() function internally.
gpt = mv.Geopoints(path='/path/to/my/geopoints/file.gpt')
Look at the Fieldset constructor as an example.