remove non-cython method and get rid of the spa class

[j-i-l]

Closes #45

Closes #44

[j-i-l]

All non-cython implementations are now exported to _cython_subst.py which we can easily drop if we want.

flowstab.SPA is now only still used in the _cython_sparse_stoch.stoch_mat_add/sub functions and called in sparse_stoch_mat.__add__ and sparse_stoch_mat.__sub__.

@alexbovet should we rewrite _cython_sparse_stoch.stoch_mat_add/sub to get rid of flowstab.SPA completely?

[j-i-l]

might need some adaptations once #49 is merged

[alexbovet]

Let's keep flowstab.SPA for the moment until we do some proper testing of it.

might need some adaptations once #49 is merged

What do you mean? can we merge this?

[j-i-l]

Let's keep flowstab.SPA for the moment until we do some proper testing of it.

might need some adaptations once #49 is merged

What do you mean? can we merge this?

The changes from #49 partially affect the same files, so we just need to update this branch and sort out the potential conflicts, then this is ok to merge.
I'll update and check it.

[j-i-l]

@alexbovet now is fine.

I'll then bring the branch for #12 up to date and the we can merge in all the testing