First implementation of the Hypnogram class (#116)

* Initial class creation for Hypnogram

* Add properties + mapping

* Add sleep_statistics and upsample

* Add mapping for all n_stages + SFI

* Fix in sleep_statistics when all WAKE

* Minor edits

* Added `scorer` and two new methods

* Add docstring

* Start adding unittest for Hypnogram class

* Add more docstring

* Add main docstring + unit test upsample_to_data

* Uncommented upsample function

* Add to API

* Add unit tests for 3-5 stages hypnogram

* Added consolidate_stages method

* First minor comments from PR review

* Initial class creation for Hypnogram

* Add properties + mapping

* Add sleep_statistics and upsample

* Add mapping for all n_stages + SFI

* Fix in sleep_statistics when all WAKE

* Minor edits

* Added `scorer` and two new methods

* Add docstring

* Start adding unittest for Hypnogram class

* Add more docstring

* Add main docstring + unit test upsample_to_data

* Uncommented upsample function

* Add to API

* Add unit tests for 3-5 stages hypnogram

* Added consolidate_stages method

* First minor comments from PR review

* Add docstring for method `upsample`

* Added as_annotations method

* Minor docstring edits

* Epoch starts at 0 in `as_annotations`

* Updated changelog

* Fix typo