Using the word 'parameter' instead of the word 'argument' in most places.

We have been inconsistent in our use of the words 'parameter' and 'argument'. Most scientists are more familiar with the first than the second, so this PR changes all uses of 'argument' in code to 'parameter'.  However, uses of the word 'argument' for command-line shell arguments are unchanged, since:

1.  that's what man pages call them, and
2.  `argv` is easier to explain if we use the word "argument".