Finish initial code refactor

[ojeda-e]

From merged PR #33 the following changes are pending:

• [x] Add a fail test with negative coordinates in grid_map.
• [x] Combine the 9-grid and the 25-grid into one function with pytest.parametrize.
• [x] Add a test where the output of grid_map is different from the input dummy_coord.
• [x] In line 187, check test_mapper. Suggested change: assert test_mapper(dummy_coord) == grid_map(dummy_coord, factor) for assert grid_map(dummy_coord, factor) == dummy_coord.

[ojeda-e]

Issue closed with PR #34.