Multitrack MUSDB18: MedleyDB Subset

A subset of the MUSDB18 dataset, sourced from MedleyDB, was reestimated to obtain the individual stem contribution. Note that this was estimated at the MedleyDB stem level and not at the MedleyDB raw level. For a MUSDB stem that consists of only one MedleyDB stem, the MUSDB stem itself is copied over directly, and the SNR is marked as "80 dB" in the table. In practice, this just means that the SNR is infinity. The mixture SNR is still the real computed value, since no tracks consist of all four stems that are singletons. For the most part, the tracks were summed over directly using time-invariant gain estimation. For some of the tracks, the bass signals were actually filtered. I suspect that there is also filtering for tracks that have an SNR lower than 50 dB on some stems as well. We will perform the estimation from the raw data later, as it's a lot more complicated.