This change adds a large part of the Graph API, including most of the low-level
Expression API for code-first runtime-focused customizing of modifiers, and an
experimental preview of the higher level editing-focused Node API.

The graph module, as well as its expr and node submodules, contain design
notes and high-level introductions to the new Expression and Node APIs,
respectively.

The CHANGELOG has been updated with an exhaustive list of changes. We describe
here the main changes only.

Notably, this change removes the InitAgeModifier and InitLifetimeModifier,
which are superseeded by the more generic and powerful InitAttributeModifier,
which can initialize any particle attribute. The CHANGELOG lists all modifiers
which have been otherwise impacted by some changes.

This change is a major breaking change, and is accompanied by a migration guide
(in the docs/ folder) due to the large amount and complexity of changes
compared to previous releases. The migration from v0.6 is relatively
straightforward however, following the guide or looking at the examples.

All examples have been converted to this new Expression API, and are fully
functional. The change should be on parity with the previous architecture;
however if you notice any discrepancy or regression please open a bug.