metrics API improvements

[lucasgautheron]

• [x] tests
  • [x] segments_to_grid tests for each combination of (overlap, none)
• [x] documentation
  • [x] reflecting overlap and none options in segments_to_grid
  • [x] reflecting the changes to conf_matrix()
• [x] implementation
  • [x] faster confusion_matrix as a matrix multiplication
  • [x] make overlap and none options in segments_to_grid