Mutwo.core

Changed
- set correct order in auto import of modules (function: `mutwo.core.utilities.tools.import_all_submodules`)

Added
- `mutwo.core.events.envelopes.Envelope.integrate_interval` function
- `mutwo.core.events.envelopes.Envelope.get_average_value` function
- `mutwo.core.events.envelopes.Envelope.get_average_parameter` function

Added
- `mutwo.core.events.envelopes.Envelope.is_static` property

Added
- `mutwo.core.parameters.abc.PitchInterval` class
- `mutwo.core.events.envelopes.RelativeEnvelope` class

Changed
- pitch classes need to define a `add` and a `subtract` method from now!

Added
- `mutwo.core.events.envelopes` module

Changed
- moved `DurationType` to `mutwo.core.utilities.constants`

Changed
- refactor mutwo code: split mutwo into `core` and `ext` packages. `core` only contains the main / shared / most important code