Fix Issues when running tests on the CI

See merge request biosignal/software/mednet!19

Histograms comme be compared directly due to differences in how the
torch.resize transform is applied depending on the environment.
A statistical approach using pearson coefficients is used instead.
Testing of alexnet and densenet transforms have been temporarily
disabled as histograms or RGB images need to be handled differently.

Databases should now be available on the CI, we do not rely on a zipped
db anymore.

Samples are not square cropped during data loading anymore due to recent
changes. Cropping is model-dependent.

By default, performs sal gen and sal view. Default parameters
are used for now. Steps requiring databases with human annotations
are not yet included in the experiment script.

Add data-model diagram

Closes #24

See merge request biosignal/software/mednet!17

Update documentation

See merge request biosignal/software/mednet!15