Add NodeDoneFlags enum in Methcla namespace
Add platform dependent API implementation for ios
Remove methcla_init hack for Xcode

Doesn't seem to be needed anymore when using Methcla subproject.
Copy libmethcla.a to simulator build location
Don't try to set audio unit buffer size in simulator

iOS simulator doesn't respect the setting of the
kAudioUnitProperty_MaximumFramesPerSlice property; the number of frames
passed to the render callback always seems to be 512.
Add new node done flag and fix Node::setDone

Node::setDone needs to test against multiple flags.