Weight logic to Knockback

[ThierryBerger]

Related to #291

Impacts would only be "BombPush" ability used on ally Heavy agents.

Features:

• Weight logic for BombPush
• Same logic for FlyOff effect
• Visualization for failed knockback
• PR for assets/objects.ron definitions : added weight to objects zemeroth_assets#6
• Tests for Knockback
• Tests for FlyOff

[ThierryBerger]

Ok tests are not passing now, there must be something not deterministic in there, sometimes is succeeds and sometimes not, I guess the simplest fix is to split the test in 2 : "heavy" and "normal".

I separated tests for "heavy" and "normal", but ❗️ there's no test to actually make sure the movement was indeed applied for "normal", but not applied for "heavy"

[ozkriff]

Could you rebase on top of the master branch (to fix Travis checks), please?

[ozkriff]

Also, this PR should be paired with a PR to https://github.com/ozkriff/zemeroth_assets/blob/master/objects.ron otherwise the actual game crashes during initialization with a 'Can't parse the prototypes: Message("missing field `weight`")' msg.

[ozkriff]

Added a few final touches.

Thanks for the PR again, @Vrixyz!!