Pymavlog

Fix

* Fields indexing: ([`a17ded0`](https://github.com/rmargar/pymavlog/commit/a17ded06670ab4e6daf25a21d7727357f165815f))

Feature

* Add max_rate_hz to discard messages based on maximum rate ([`019ea3a`](https://github.com/rmargar/pymavlog/commit/019ea3a4f33a4a6db6b13230a90232f12a5cf3d5))

Feature
* Add columns property to series ([`9c8654b`](https://github.com/rmargar/pymavlog/commit/9c8654b457e9598dccd5a116bb8d79906170b581))
* Make both mavlog and mavlink series objects subscriptable ([`d8b1d29`](https://github.com/rmargar/pymavlog/commit/d8b1d2931ef569c4a87ad4a06391ecf32bc0b98d))

Documentation
* Add example for accessing subscriptable values ([`a06c000`](https://github.com/rmargar/pymavlog/commit/a06c000ea388119267434f9c1cc0c32ee97121cf))

Feature
* Add mavtlog class ([`f3c3153`](https://github.com/rmargar/pymavlog/commit/f3c3153c361d49041c72737654efc455def2cf7d))

Documentation
* Include small example about using mavtlog ([`f15b8cf`](https://github.com/rmargar/pymavlog/commit/f15b8cff5866e203f8f4aad7c02d1e570493eda7))