Changelog:
* New `Subject` class models a chart native or partner, and accepts a date and coordinates.
* Chart classes now only accept an instance of `Subject` instead of direct dates and coordinates.
* Chart classes no longer return date & coordinates properties, and instead return native / partner properties of type `Subject`, which contain the date & coordinates.
* Composite MC-based house calculation has been removed due to being overly complex and error-prone when dealing with house-based objects. Composite chart is now 100% midpoint-based.
* Synastry chart has been removed, and instead a new `aspects_to` parameter has been introduced into each chart's constructor, which accepts a chart instance of any type and calculates the main chart's aspects to it. Synastries now consist of two separate chart instances, one of which aspects the other.
* `Transits` chart has been introduced, which is an easy shortcut to a natal chart of the present moment.