initial renames & tests for three important sandbox scripts

These three scripts (originally sandbox/strip-and-split-for-assembly.py, interleave.py, and split-pe.py) are widely used in our tutorials and protocols.  Time to move them into scripts/ and test 'em!

The first commit here is the initial rename and the initial tests.

Quite a few things remain to be done; just off the top of my head,
 - argparse usage
 - FASTA and FASTQ both: scripts that take in FASTQ should output FASTQ, etc.
 - tests for all of these things
 - documentation for the scripts