Document better the new Extended TM parameters apical/basal input dim

[breznak]

With the new update, I found ETM requires new initialization parameters:
https://github.com/numenta/nupic.core/blob/master/src/nupic/experimental/ExtendedTemporalMemory.hpp#L96

There's no documentation for them, nor reasonable defaults... I searched in examples and tests.
Please provide a better explanation & suggested default values.

[rhyolight]

@subutai @mrcslws I would like to clarify the line between research and production code. I see there is now an experimental folder in nupic.core, but not in nupic.

I can understand why you might do this to do C++ research without adding the make complexity to nupic.research. Is that the case here? If so, I think you should an experimental/README.txt that clearly defines this directory as research with the normal disclaimers.

[mrcslws]

Yeah I agree that the research code could be better separated from the public API.

(FYI there is already a experimental/README.md, but maybe it should be longer.)