Add some well known dimensions to cusfsum.Case

It would be convenient if `cusfsim.case` could have some constants for well known units. E.g.:

``` python
# inside case.py
class Dimension(...):
KILOGRAM = Dimension(1, 0, 0, 0, 0, 0, 0)
NEWTON = Dimension(1, 1, -2, 0, 0, 0, 0)
PASCAL = Dimension(1, -1, -2, 0, 0, 0, 0)
```

Etc, etc. Look in the [sphinx documentation](http://sphinx-doc.org/ext/autodoc.html#directive-autoattribute) for how to document class attributes.

One may need to do

``` python
Dimension.KILOGRAM = Dimension(1, 0, 0, 0, 0, 0, 0)
```

rather than the method above. This depends on whether the `Dimension()` constructor is available to class attributes.

After this, one could refer simply to `Dimension.PASCAL` in scripts which is a little less obtuse :).