Strategizer for multithreaded enumeration

[cr-marcstevens]

DO NOT MERGE YET

• further testing is suggested
• uses feature branch of fplll

Changes:

• added enhanced version strategizer_walltime.py
• allow setting # enumeration threads for set_mdc.py and strategizer_walltime.py with -e <N>
• allow using walltime instead of cputime as optimization metric in strategizer_walltime.py with cmdline option -W
• updated to current branches fplll/simple_parallel_enum and fpylll/master

Also I'm aware current master links to specific fplll/fpylll commits for reproducability reasons.
How do we want to handle these kinds of improvements?
First copy master into some archival branch ?