SceneGraph: FeatureGroup compilation optimization.

Created less-templated base for FeatureGroup and moved its
implementation into *.hpp file, allowing to remove <algorithm> header
from FeatureGroup.h (and meanwhile removing forgotten one also from
AbstractGroupedFeature.h). Saved another ~10 seconds of compilation time
(previously ~4:21, now ~4:11).
SceneGraph: AbstractFeature compilation optimization.

Constructor, destructor and all virtual functions are moved into
implementation file, so they don't need to be recreated and add into
binary on every usage. Should save a bit on resulting file size.
SceneGraph: removed superfluous instantiation sources.

Moved almost everything into one file, instead of compiling ~50k LOC
seven times it is now compiling 56k LOC only once. This saves another ~5
seconds of compilation time (before ~4:11, now ~4:06).

Also explicitly saying that we are instantiating Float version of all
classes. In the future we might have compile switch for building also
Double one, this helps with consistency.
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