Add ZMAP write support for earthquake catalogs

More formats are likely to follow later. Let me know if you need me to make any changes.

Best
Lukas
This is by design (see also tests/documentation). The main issue here is that there is no way to identify magnitudes in zmap since it's a purely numerical format (no headers etc.). If some events in the catalog have an Mw and others have an Ml, the two might end up in the same column. So we export the preferred magnitude only, assuming the user knows what it is. The same goes for origins.