Change return type of frame cell to Option

[seatonullberg]

Many file formats do not always store a bounding box. I think it would be a significant usability improvement if getting the cell from a frame behaved the same way as getting the velocities (#25).

Currently, if the frame does not contain a cell it will silently return a default cell that is initialized with all matrix elements set to zero. In order to check for this one must iterate over the matrix elements and assume that a matrix full of zeros means the cell was not found in the file.

I would be happy to make a pull request that addresses this from the Rust side if it is something that doesn't need to be handled in the underlying C++ code.

[Luthaf]

The way this is handled in chemfiles is with the CellShape enum, where an Infinite cell correspond to non-periodic systems. Would checking this first work for you?

We could also change the return type of UnitCell::matrix() to be an option, returning None when the cell is infinite.

[seatonullberg]

Thanks for the clarification. Checking the CellShape enum will work for my purposes but I also like the idea of returning an option from UnitCell::matrix(). To me that seems like a more rust-y approach than returning a default value.

[Luthaf]

but I also like the idea of returning an option from UnitCell::matrix().

Since this will be a breaking change, I'd rather do it after the release for 0.10.4.