Make Python plan attributes private

[janden]

Makes all the attributes private by underscoring their names. They can still be accessed in the public interface, but only as (read-only) properties. This applies for both CPU and GPU code.

Fixes #565.

[DiamonDinoia]

if we use @Property do we need to use _dtype internally in read-only the class? AFAIK if we are reading _dtype, dtype would be enough.

[janden]

We can do that for all of these properties. Only issue is that the code becomes slightly ugly in places where we need to access both public and private attributes:

        ier = self._make_plan(self.type,
                              self.dim,
                              _n_modes,
                              self._isign,
                              self.n_trans,
                              self._eps,
                              byref(self._plan),
                              self._opts)

[DiamonDinoia]

I do not have a preference. I will trust your judgement. Once the merge conflicts are solved. I will merge the version you deem more readable.

[janden]

Ok. I think I'll revert the last two commits then and rebase. Once all the tests pass, feel free to merge.