Automatic Cell

[cortner]

consider redefining the cell in terms of the extent of X when NOT PBC

[cortner]

more generally, make it a default to ignore the cell dimension in the non-PBC direction and always project to the cell in the PBC directions. + document this clearly. That way there is no more ambiguity how the positions are treated. See also #6

[cortner]

While I am revisiting the meaning of the cell variable, should also double-check and test what happens when the determinant is negative. Probably it makes no difference, but who knows?

[cortner]

this is implemented by still needs to be documented