Create a persistence diagram type

[raphaelreinauer]

Issue:
When using persistence diagrams as pytorch tensors one always has to check if they
the input has the right format (2 dimensional, one-hot-encoded homology typ ...) which
is a huge overhead.

Suggestion:
We could use the typing module to define a subtype of tensor which represents
one-hot-encoded persistence diagrams. This would allow to stafely pass around
persistence diagrams without having the overhead of additional assertions everywhere.

Sample code:

from typing import NewType

# Create a subtype of tensor representing one-hot persistence diagrams with`
# almost zero runtime overhead.`
OneHotPersistenceDiagram = NewType('OneHotPersistenceDiagram', torch.Tensor)`


def one_hot_persistence_diagram(persistence_diagram: torch.Tensor) \
    -> OneHotPersistenceDiagram:
    """Convert a persistence diagram to one-hot encoding."""
    assert persistence_diagram.ndim == 2
    # ..... additional assertions here ...


    return OneHotPersistenceDiagram(persistence_diagram)
x = torch.tensor([[0.0, 1.0, 0.0, 1.0],
                  [0.2, 0.4, 1.0, 0.0]])

pd = one_hot_persistence_diagram(x)

def get_number_of_homology_type(persistence_diagram: OneHotPersistenceDiagram) \
    -> int:
    """Get the number of homology types in a persistence diagram."""
    return persistence_diagram.shape[1] - 2

get_number_of_homology_type(pd) # = 2
get_number_of_homology_type(x)  # Will cause a TypeError

 # Otherwise we can use pd as a usual pytorch tensor