pyTMD: Python-based tidal prediction software

Summary: This release adds a couple of fixes for issues found by both @andrew-longmore-tessella and @torbjoernt. In one issue, the variable parsing wasn't working as intended for FES-formatted files for tidal currents. In the other issue, the units were not being handled properly for some ascii-formatted models, which caused issues when trying to convert to default units (e.g. from cm to m).

This release also adds some minor features, such as an improved representation of pyTMD.io.model objects when using Jupyter notebooks.

Itemized Changes:

• refactor: make model io accessors sub of tmd accessor (#520)
• ci: change joss paper workflow to manual dispatch (#521)
• fix: do subaccessors properly with registration (#522)
• feat: add HTML representation for io.model using xarray (#523)
• fix: determining FES current variables for #524 (#523)
• fix: set units on constituent variables in a loop for #526 (#523)
• fix: some GOT-formatted models have units on the second line of header (#523)

Full Changelog: https://github.com/pyTMD/pyTMD/compare/3.0.2...3.0.3