Goodman-focus

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- Implemented Github Actions
- Removed astroconda channel from environment

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- Fixed version of `ccdproc` to `1.3.0.post1`. `ccdproc==2.0.0` does have some
problems reported on `astropy/ccdproc699`

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- Changed Sigma Clipping iterations from 1 to 3
- Added sigma clip iterations as argument to function `get_fwhm` though this is
not exposed to the user.

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- Changed logger setup
- Moved data directory validation from instantiation to execution.

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- Fixed bug on the calculation of the pseudo-derivate used to find best focus
value
- Updated hardcoded string that defines the Imaging wavmode from `Imaging` to
the new `IMAGING`.
- Added docstrings

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- Created dedicated documentation for readthedocs.
- Fixed bug where return was missing,
- GoodmanFocus need to be instantiated only once [19]
- Calling instance of GoodmanFocus can receive a list of files as input [19]
- Argument --file-pattern is now actually used in file selection [18]
- Eliminated some warnings.
- Included plots in documentation.