atooms / atooms

Python should not increase minor numbers with backward incompatible
changes. Unfortunately it does.

Closing xyz files resets the numpy string function. If this occurs
between two calls to write_sample(), this could screw up
formatting. To prevent this, we set the string function at every
call. This may incur in a slight overhead, but at least it is safe as
long as we stay single thread.

Do not know why, but sometimes '' is not there.

Also we print errors in custom trajectory imports, because they are
useful for debugging.

This fixes the case when the suffix is already present.

The fields attribute was supposed to store only particle properties, not system ones.

Fields are meant to be particle attributes, not system objects. We may
need an extra variable, or a more general schema descriptor, to
signify to other classes what objects and/or fields are written in the
trajectory.