Add Lowell Tilt Current Meter (TCM) support

[ssuttles-usgs]

Add support to process Lowell Tilt Current Meters. New tcm.py module created to process current data, tests, and documentation have been updated. Added LFS tracking for files in tests/data folder.

[dnowacki-usgs]

Thanks Steve! I'm wondering why for this instrument we are publishing speed and direction when for most other instruments we only publish east and north components. Also, if we do indeed want to publish spd/dir, it looks like speed and direction are already provided by the file. Why recompute it?

[ssuttles-usgs]

Hey Dan, Welcome back! I anticipated the first question. main reason that speed and direction are included is the instrument software includes it, as you point out, and I thought some users might want/expect it. I went back and forth on that one. The reason for recalculating speed and direction is because need to apply the magnetic variation. I guess I could have just adjusted the direction for it, but I decided to find from east and north after being rotated for magnetic variation.