Decode

- Update version numbers written in:
- pyproject.toml
- decode/__init__.py
- docs/conf.py
- tests/test_metadata.py

- Use Poetry for package management
- Use GitHub Actions for testing and publishing package
- Update version numbers written in Python scripts

- Remove joke functions
- Use Travis CI to do auto testing and docs building
- Docs are now hosted in the `gh-pages` branch

+ `plot_chmap` now returns mappable and does not plot colorbar inside it
+ Delete default styles of xlabel, ylabel, and title of plot functions
+ Fix `plot_timestream` (before that an error occurs when plotting)

Removed
+ `dc.models.skewgauss()`
+ `dc.models.r_subtraction()`
+ `dc.models.savgol_filter()`
+ `dc.models.rsky_calibration()`
+ `dc.plot.plotweather()`

Implemented
+ `dc.models.gauss_fit()` made by Koyo
+ `dc.fromcube()` made by Akio
+ `dc.plot.plot_chmap()` made by Tsuyoshi

Updated
+ `plotcoords()` => `plot_tcoords()` (backward compatibility has been held)
+ `plottimestream()` => `plot_timestream()` (**backward compatibility has been lost**)
+ `plotspectrum()` => `plot_spectrum()` (backward compatibility has been held)
+ `savefits()`
- casaviewer has been supported
- still should be upgraded

Miscellaneous
+ implemented `deprecation_warning` decorator
+ changed some of the internal structures
+ made the logger fucntions a standard way