Non-negative CP + warm-start CP

[scttfrdmn]

Two additions to decompose.py: non-negative CP (multiplicative updates) and warm-start CP accepting an initial factor set. Helps downstream users converge faster on repeated calls.

[scttfrdmn]

Implemented in c69fc7e: nonneg=True (multiplicative updates) and factors= (warm-start) kwargs on cp_decompose().