Fix typo.

Rename nAtOne to nAtOnce.
Add nJobs check.

If HasMP is false, then prevent nJobs from being set higher than 1, even
if the user passes the -j flag.
Fix bug when only one input file is given.

Ahadd now runs a simple copy operation if only one input file is given.
Previously it threw a "Not enough arguments" error, but this is often
undesirable if ahadd is run as part of a script where the number of
files it will have to deal with is unknown.
Switch to #!/usr/bin/env python

Hardcoding in #!/usr/bin/python prevented users from using their own
version of python. Changing to #!/usr/bin/env python which will grab
python from the users path, allowing them to use an updated version
(that supports multiprocessing, for example) if they want.
Fix a typo in the copyright statement
Fix typo in options
Finxing typo in the README
Spell "quiet" correctly