Revamp training/testing divide

[sampsyo]

Overhaul the feature where we distinguish between the training and testing inputs:

• [x] A more elegant interface than duplicating the Makefile: some more friendly way for programmers to write the alternate input command line (or potentially multiple alternates).
• [x] Should work with memoization. Currently, training and testing are conflated in the memo table.
• [x] Avoid hacking the cwmemo library, which is supposed to be abstract.

[sampsyo]

Mostly in place. Outstanding issues:

• [x] Add this to the experiments (exp) command as well as run.
• [x] Add testing inputs to all the benchmarks (obsoleting Makefile2).
• [x] Different number of reps for training and testing.
• [x] Documentation.