Goodman-focus

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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.

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- Added messages when no file matches the `--obstype` value, by default is
`FOCUS` [9]
- Replaced `parser.error` by `log.error` and `sys.exit` when the directory does
not exist and when exists but is empty.
- Added test for cases when the directory does not exist, when is empty and when
no file matches the selection on `--obstype` which by default is `FOCUS`.
- Replaced `logging.config.dictConfig` by `logging.basicConfig` which fixed
several issues. For instance `--debug` was unusable, and also there were
duplicated log entries for the file handler when used as a library in other
application. [10]
- Replaced the use of the function `get_args` by using arguments on class
instantiation instead
- Created name for modes [11]