**Deprecation**: clib.Session.virtualfile_from_vectors now takes only one argument

[seisman]

This is a POC PR.

In the Session.virtualfile_in method, we must prepare for _data that will be passed to different virtualfile_from_* methods. Currently, _data must be an iterable (e.g., _data = (data, ) even for the simplest case) and when pass it to _virtualfile_from, we need to unpack it like:

_virtualfile_from(*data)

The complexity is because, while most _virtualfile_from_* methods only take a single argument, the _virtualfile_from_vectors takes multiple arguments. The _virtualfile_from_vectors is defined like _virtualfile_from_vectors(*vectors) and must be called like _virtualfile_from_vectors(x, y, z).

This PR changes its definition to _virtualfile_from_vectors(vectors), and now it should be called like _virtualfile_from_vectors((x, y, z)).

With this breaking change, the virtualfile_in function can be simplified slightly, which I think is more readable. Then the question is, is it worth a breaking change?

Edit: Actually it's possible to introduce the new syntax without breaking backward compatibility. This is done in f5b0e8e.