merge descr with external metadata

[diemoschwarz]

• related to #13
• extension of catart-mubu-tutorial 13 import external descriptors (#104)

Use Case

• [x] 1. load corpus from text file with filenames and externally calculated metadata as in catart-mubu-tutorial 13 import external descriptors
• [x] 2. calculate descr and add/prepend to metadata
  • [x] in extra track, then combine tracks?
  • [ ] in place in loaded track?
• [x] 3. allow recalc of descr in place?

[diemoschwarz]

needs track merge:

• practical solution: mubu track js
• alt 1: mubu message mergetracks ?
• alt 2: mimo module merge (needs possibility to specify multiple input tracks for mubu.model)