Implementation is moved into private virtual `do*()` functions and
public interface does additional sanity checks around them. Exporting to
file is by default done in base implementation, which takes exported
data and then saves them to the file.
Added unit test for file export, bumped plugin interface version to 0.2.
Implementation is moved into private virtual `do*()` functions and the
public interface does additional checks aroung them to simplify plugin
development. Opening files is by default done by the base
implementation, which then calls function for opening raw data with file
contents.
Added test for file opening, bumped plugin interface version to 0.3.
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Vladimír Vondruš