Fix SofaBaseTopology module name and import path

[jnbrunet]

#108 changed the module name SofaBaseTopology to BaseTopology but did not propagate the change inside the init function of the module, which created a linked error when importing the module (I'm looking at you, @fredroy).

This PR fixes this, and also move the module into the Sofa package. Now, instead of doing

import BaseTopology

which doesn't make it clear that this is a Sofa package, you do:

from Sofa import BaseTopology

[damienmarchal]

Thank @jnbrunet for the PR.
@marques-bruno can you look at it please (and the CI with the +2 failures)