Robot animation fixes

The MvM robot models are lacking many animations that are not normally used by robots in MvM, or which have been added to the game more recently than the Mann vs Machine Update.

This mod changes the robot models so that if they are missing a particular animation, they will fall back to the regular human animation for their class, instead of having no animation at all.

The engineer bot model already does this (apparently due to Valve being too lazy to give the engiebot a full set of custom robot animations). My mod just extends the same "fallback to human animation" behavior to the rest of the robot models.

This mod works for all classes, giant and non-giant. And it will continue to work in the future, even for animations that haven't been added to the game yet, because each model file simply references the file with the human animations, rather than copying any of its contents.

Note about sv_pure

This mod will not work on sv_pure servers (including Valve servers). It will only work on servers that don't enforce sv_pure on the files that are modified.