Python round() to numpy.round()

`round()` changed between python 2 and 3

**Problematic outcome**
A testcase with different axes for python 3 compared to python 2
and so could not agree across versions.

**Solution**
`numpy.round()`
Specify the default aesthetics for the geoms

**stuff done**
- geom.DEFAULT_AES a dict with the default values for the
  aesthetics
- geom.plot_layer to handle per geom manual aesthetics and
  have them override those in ggplot.
- save test result images that are more representative of
  of the desired outcome.