Implement multiprocessing support for CachedDataSource

Closes #32

See merge request !71

These test ensure that DataLoaders can be used with
the multiprocessing module if these are passed through
a queue.

This will allow to pass an instance of it in
a queue when using the multiprocessing module.

This will allow to pass an instance of it in
a queue when using the multiprocessing module.

This allows to reset the state of a data source.

Currently only used by the CachedDataSource

Improve file name handling and removal in CachedDataSink

See merge request !70

This ensures that all files are tentatively removed.

Logger cleanup

See merge request !69

Python provides better tools that exceptions for that.

Fix wrong instance variable name in error message

See merge request !68

Fix wait method

Closes #30

See merge request !67

Improve CachedDataSink error messages

See merge request !66

If the opening of the cache file fails, print an error
message to know where to look at.