v0.16.0: Add BasicCallableMapping abstract base class (#119)

Add new class to module `sympde.topology.mapping`:
   - Require methods `__call__`, `jacobian`, `metric`, `metric_det`, all called with arguments `(*eta)`
   - Require properties `ldim` and `pdim`
   - Require method `jacobian_inv`, which should raise exception if Jacobian matrix is not invertible (e.g. when `ldim < pdim`)
   - Make `CallableMapping` subclass it
   - Spline and NURBS mappings in Psydac will subclass it

Add new method `set_callable_mapping` to symbolic `Mapping` class, in order to attach any object of type `BasicCallableMapping` to it. Psydac will use it to attach spline and NURBS mappings to MappedDomains with undefined Mapping

Further changes:
   - Allow for mappings with `pdim > ldim` (fixes #70)
   - Update library version to 0.16.0