Bug: a88/9a0

So far two users (Christian and Bruce McFadden) have expressed a
need to use PuffinPlot for simple directional data (presumably 
sample directions obtained from a different program). PuffinPlot 
doesn't currently allow this: even the custom import dialog will
read every line of the file as a demagnetization point. I can 
envisage a somewhat unpleasant hack involving flipping directions,
anchored PCA to "promote" step directions to sample directions, etc.
but it's probably better to do it properly. Shouldn't be too hard
but there are probably a few places in the code where I assume that
there will be at least one demagnetization step associated with a
sample.

Now implemented in a simple fashion, though quite a lot of changes were
required even for this basic functionality. PuffinPlot now supports:
importing of directions to create samples without any demagnetization
data; plotting and analysis of imported directions in the same way as
"normal" (PCA and Fisher) sample directions; and saving and loading of
these directions in PuffinPlot files.

There's one notable omission: because sample and formation corrections
are applied at a Datum level, they have no effect on imported sample
directions. Fixing this would (I think) be a fairly large and delicate
job: the existing data model has to be kept for "normal" data, and the
corrections applied at the sample level *only* for the imported data.
It's not clear whether either of the original requesters actually
required this functionality, so for now I'm omitting it. Can always
open a new ticket and implement it if it's requested later.