[BUG] surface points added to wrong surface

[Japhiolite]

Description

Manually added surface points with add_surface_points were allocated to the first created surface.
This was due to a type check solely for int. Changed to isinstance(idx, (int, np.int64)) as somewhere along the creation, idx gets changed from int to np.int64...

Relates to #813

Checklist

• My code follows the PEP 8 style guidelines.
• My code uses type hinting for function and method arguments and return values.
• My code contains descriptive and helpful docstrings
  which are formatted per the Google Python Style Guidelines.
• I have created tests which entirely cover my code.
• The test code either 1. demonstrates at least one valuable use case (e.g. integration tests)
  or 2. verifies that outputs are as expected for given inputs (e.g. unit tests).
• New and existing tests pass locally with my changes.