made grad_sources_inputs accept None for inputs

types

variables to marks

marked some tests as KnownFailure--they fail due to unconventional
Op.grad implementation

implemented an unimplemented gradient case, this got rid of most of the
KnownFailureTests this branch introduces

still run it to compare against new version of grad)

tests harder

just one way

fixed bug; cost was considered a disconnected input

Add proposal of math framework for complex grad

No gxx

More things work when g++ is not available.

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Added Tutorial on Python's memory management