Ability to create score excerpts [FEATURE]

[johentsch]

The low-hanging fruit is to allow stripping all tags that are not included in the desired excerpt.

ToDo

• [x] implement the low-level _MSCX_bs4.make_excerpt() #90
• [x] implement a high-level method such as MSCX.store_excerpt() with a user-friendly interface
• [x] more convenience methods, e.g.
  • [x] excerpts for all annotated phrases
  • [ ] excerpts based on a sequence of start measures

[johentsch]

addressed via #91