Importing Keras when loading dataset

[swapsha96]

I just observed that when I am using any function from mirapy.data.load_dataset, the module always runs from keras.utils.np_utils import to_categorical which is only used in one function. So if a user just wants to load dataset and doesn't have keras installed, it will throw an error. This is a utility that needs to be replaced by either creating our own to_categorical function or find a better way.

Note: Once the assignee creates the PR, s/he needs to check if notebook on X-ray binaries is working correctly.

[swapsha96]

I will suggest using OneHotEncoder in sklearn to convert into categorical values.