Track relevant echopype issues

[emiliom]

This may be redundant, but let's see if having this tracking issue here is helpful:

• Separate out functions that bring data from processing level 0 to 1 echopype#581

[leewujung]

I think so! This will help us put together the processing chain/pipeline.

Adding a few here:

• Accepting and handling external-data provenance information echopype#499
• Add mechanism to convert echo_range to depth in calibrated datasets echopype#578
• Add mechanism to interpolate lat/lon to Sv data echopype#725
• Add function to regrid Sv data echopype#726

[emiliom]

• Add global attribute for data processing level echopype#980