Issue/635/Modify coordinate system implementation

[marina-ricci]

This PR aims at resolving issues explained in issue #635.

Copied from the issue :

These are my suggestions :

• [ ] add warning message when GalaxyCluster (or compute_tangential_xxx, see second point) object is initiated without specifying the coordinate system. Like "no coordinate system provided, taking euclidean as default"
• [ ] put coordinate system as a keyword for compute_tangential_and_cross_components, from data_ops or as a GalaxyCluster method. Maybe not necessary to have it as a GalaxyCluster keyword because it may be confusing as it is not necessarily propagated.
• [ ] create a simple notebook to highlight this.