pyqg/pyqg: Add support for online parameterizations

What's Changed Drop support for EOL Python 2.7 by @hugovk in https://github.com/pyqg/pyqg/pull/258 Support xarray concatenation of simulation snapshots across time by @asross in https://github.com/pyqg/pyqg/pull/254 Typo: of to on by @SalahKouhen in https://github.com/pyqg/pyqg/pull/247 Add more verbose ImportError when Xarray not found by @rabernat in https://github.com/pyqg/pyqg/pull/260 Edits to docs (incl. examples) to note xarray requirement by @MFJansen in https://github.com/pyqg/pyqg/pull/261 Fix units of pv quantities in variable database by @cesar-rocha in https://github.com/pyqg/pyqg/pull/269 Add support for online parameterizations / custom forcing functions by @asross in https://github.com/pyqg/pyqg/pull/266 New Contributors @hugovk made their first contribution in https://github.com/pyqg/pyqg/pull/258 @SalahKouhen made their first contribution in https://github.com/pyqg/pyqg/pull/247 @MFJansen made their first contribution in https://github.com/pyqg/pyqg/pull/261 Full Changelog: https://github.com/pyqg/pyqg/compare/v0.4.0...v0.5.