Bug: a88/520

Requested by Christian after experiences on the JOIDES. Fundamentally,
all that's needed for analysis is a series of 3-dimensional vectors,
so it's a shame that Puffin is tied to the extremely specific OPRF/2G
input format (complete with misspelling of `Azimuth'). Puffin should
accept another, stripped-down format which could be as simple as x/y/z 
but should also have room for e.g. sample code and demag level. This
would also force me to think about what fields are actually necessary in
the full 2G files, allowing malformed files to be rejected on loading
rather than causing a crash at the display stage.

Get files in other formats off Christian for testing.

No sense doing a release if we can only read OPRF files! There should be
a well-defined set of required input fields in the documentation.

Have discussed with Christian. We thrashed out the notion of a
preview window for tabulated data of unknown format, which would allow
the user to specify which columns meant what. This is technically
straightforward, but will require some careful UI decisions.
Christian has put the word out for examples of other data formats,
so I should have something to work with soon.

Interpreting long core files requires knowledge of the effective
sensor lengths, so this will have to be moved to a configuration
option.

Most of the heavy work is done on this now, so I'm going to split
off the remaining bits into separate bugs and close this.

Mostly implemented, remaining bits opened as more specific new bugs.