Added Basic Lat/Lon/Time Scaling + GeoPhys Vars

[jejjohnson]

NOTE: Don't merge until #32 is merged.

TLDR: I added some basic rescaling for lat/lon and time coordinates.

• The lat/lon converts the lat/lon to meters with optional rescaling.
• The time converts from datetime[ns] to a specified frequency and unit with an optional start coordinate.

Note: I had the unscale (for time) but the units don't match. A related issue: link.

---

Features

• [X] rescale time coords from datetime64 to user specified units, e.g. seconds, hours, days, etc. (check docs)
• [X] rescale lat/lon coords from degrees to meters
• [X] Key vars (u,v-velocity, stream function, kinetic energy, relative/absolute vorticity, divergence, Strain, Okubo-Weiss)
• [X] everything is units aware with pint (should probably use this with the lat/lon/time checks)
• [X] Tests

---

Also added geophysical variables that are related to sea surface height. Related to #17