Create local headers in FileView serialization

Inspired by [zip's format](https://en.wikipedia.org/wiki/Zip_(file_format)), it might make sense for the offset, linear, quadratic, and variable labels (see #659) to have their own local headers. The offsets could also be stored in the header for easier reading, rather than being implicitly derived ([example](https://github.com/dwavesystems/dimod/blob/7bec06d6c868efef012b817ffafb5043440ffd8d/dimod/serialization/fileview.py#L168), [example](https://github.com/dwavesystems/dimod/blob/7bec06d6c868efef012b817ffafb5043440ffd8d/dimod/bqm/adjarraybqm.pyx#L481)).