Fixing: C class 'SOL' is declared but not defined

[tonetto]

After a clean python2.7 install (anaconda), python setup.py build was returning this error. The solution was to simply move the attribute declarations from the .pyx file into the .pxd.

Reference: https://stackoverflow.com/questions/17735051/share-extension-types-in-cython-for-static-typing?utm_medium=organic&utm_source=google_rich_qa&utm_campaign=google_rich_qa

[tonetto]

This fixes issue #9 .