lhef 0.3.3

Read and write files in the Les Houches Event File format
Documentation
# Version 0.3.3

- Improve performance by a lot
- Fix documentation
- Update `itertools` dependency

# Version 0.3.2

- Update dependencies
- Address compiler warnings (Rust 1.46.0)

# Version 0.3.1

- Added license file

# Version 0.3.0

- Update to Rust 2018 (1.35.0)

# Version 0.2.0

- Implemented a writer
- Support attributes in xml tags
- Renamed reader `event` method to `hepeup`
- Separate `xml_header` method for reading xml headers
- Added changelog