Merge opencog/opencog to singnet/opencog

[ngeiswei]

Contains PRs:

• https://github.com/opencog/opencog/pull/3564
• https://github.com/opencog/opencog/pull/3565
• https://github.com/opencog/opencog/pull/3566
• https://github.com/opencog/opencog/pull/3568
• https://github.com/opencog/opencog/pull/3569
• https://github.com/opencog/opencog/pull/3570
• https://github.com/opencog/opencog/pull/3571
• https://github.com/opencog/opencog/pull/3572
• https://github.com/opencog/opencog/pull/3567
• https://github.com/opencog/opencog/pull/3573
• https://github.com/opencog/opencog/pull/3574
• https://github.com/opencog/opencog/pull/3575
• https://github.com/opencog/opencog/pull/3576
• https://github.com/opencog/opencog/pull/3578
• https://github.com/opencog/opencog/pull/3580

[ngeiswei]

Could someone from the SP team take a look at that PR? I haven't merged anything to singnet in a while and I want to be sure I'm not screwing anything up.

Note that PLN and miner unit tests are (automatically) disabled till this PR https://github.com/singnet/atomspace/pull/106 gets merged.

[ngeiswei]

I've re-introduced .gitmodules.

The PR looks good to me, but please still have a look at it, thanks.