Add CSS format export of Inventories.

This is a fairly simple export of the `Inventory` class to the CSS format. It only covers the `affiliation`, `network`, `site`, and `sitechan` relations. The `sensor` and `instrument` relations seemed relevant, but are not implemented at this time, since I didn't need them.

The standard is a bit vague on the format of the `lddate` fields. I have picked something that is hopefully unambiguous (if UTC is assumed), while still fitting the space allotted.

Alas, I have no real test files. The tests given here just match the default result from `read_inventory`. They appear to follow the spec and are able to load in `mineos` (which is my target). If someone has Antelope or rdseed, it might be useful to cross-check against them.
Maybe a `TypeError` is better suited here? Do you think we should create some ObsPy Errors in the core module and just inherit from them all the time? Might be nice but that is probably a bigger undertaking...right now our error handling is honestly all over the place...