Query epochs by category name

Here we implement the ability to access the information in categories.xml from the Reader class with Reader.categories(). We also enable indexing of epochs by category name. In segmented and averaged MFF files, categories and epochs are associated such that each category corresponds with an epoch. We implement combining of categories information with epochs with Epochs.associate_cateogories(), which populates Epoch.name with the category name for each epoch. This is automatically called when we run Reader.epochs(), allowing us to query epochs by category with Reader.epochs['category name'].