Only use currently is to load a bounding box

Makes it easier to change cell type without needing to change the input file

Also adds functionality to determine the cell type from a string and check if it is supported

This class checks the file extension is valid, and if it isnt, exports as step by default. More file formats will be added

Will present an error if they do not exist. This means no time is wasted generating the lattice just for it to not be able to be saved