Correct typo's and comment.
Document and augment the reverse-case in can_translate method.

In the end there was little wrong.  It had appeared that UTDATE was
being tested in the from-FITS direction, when it was in a strange
format: day of week, month day, year.  In fact it was the commentary
that was unclear.  So I reinvented the tests and only realised that I
had done so upon preparing this commit.
Add extra commentary to the tests.

These comprised blank lines in the report to divide the tests for each
instrument's headers, and new headings to indicate when the forward
and reverse (to FITS) cases come under scrutiny.
Use sentences in the prologue and comments.