Stempeg

The seeking issue (21) was not fully fixed. This release should address the remaining issues when using the chunked loading using very small float numbers as start parameter

Fixes a bug (18) that occurs when `start` or `duration` is using very small float numbers (1e-6) that are literally converted into strings maintaining the scientific notation.

Also addresses 21 and add an additional check for ffmpeg and ffprobe before actually reading any files

* a demo track is now part of stempeg for convenience

python
stem, rate = stempeg.read_stems(stempeg.example_stem_path())

* added seeking
* added the ability to provide file info to reduce the number of calls

There was a bug in the earlier versions of stempeg that didn't respect the set `out_type` in the stem reader. This was fixed and the output defaults to `np.float64`.

Thanks to

hexafraction

Add some code and warnings to detect the ffmpeg version and warn users when a version older than 3.0 is used since that is adding additional silence to the output files when encoding.

Also addressing 3