Remove pyitlib dependency in order to master (and possibly accelerate) entropy computations

[smarie]

Currently we use the following functions in pyitlib:

• entropy: https://github.com/pafoster/pyitlib/blob/689797f075d189a06b2ab3ba004004d911320cb0/pyitlib/discrete_random_variable.py#L3718
• entropy_conditional: https://github.com/pafoster/pyitlib/blob/689797f075d189a06b2ab3ba004004d911320cb0/pyitlib/discrete_random_variable.py#L3291

It would be great to include functions directly in our code so as to not rely on an external dependency "just for 2 functions". Also another interesting thing is that we could compile it, for example as shown here: https://gist.github.com/kudkudak/dabbed1af234c8e3868e

This migh tbe related to #29

[smarie]

Also another reason for embedding these, would be to better control how nans are handled.
See #34