Basic IO methods/classes

Regular users may only need get_sile which retrieves the correct sile based on parsing the filename.

Retrieval methods and warnings

`get_sile`(file, args, *kwargs)	Retrieve an object from the global lookup table via filename and the extension
`add_sile`(suffix, cls[, case, gzip])	Add files to the global lookup table
`get_siles`([attrs])	Retrieve all files with specific attributes or methods
`get_sile_class`(filename, args, *kwargs)	Retrieve a class from the global lookup table via filename and the extension
`SileError`	Define an error object related to the Sile objects
`SileWarning`	Warnings that informs users of things to be carefull about when using their retrieved data
`SileInfo`	Information for the user, this is hidden in a warning, but is not as severe so as to issue a warning.

Base classes

All sisl files inherit from the BaseSile class. While ASCII files are based on the Sile class, NetCDF files are based on the SileCDF and finally binary files inherit from SileBin.

`BaseSile`	Base class for all sisl files
`Sile`	Base class for ASCII files
`SileCDF`	Creates/Opens a SileCDF
`SileBin`	Creates/Opens a SileBin