Doesn't load full SVD beam to save memory.
Reverting "Doesn't load full SVD beam to save memory."

Too slow for so many file I/O.
This reverts commit 184b518ec1d959307cc5cacfbe717a80178420f5.
New parallel_map function.

To avoid too large objects pickle error:
SystemError: Negative size passed to PyString_FromStringAndSizeNegative size passed to PyString_FromStringAndSize
Add some output info.