Add **adata and **bdata

[craigsapp]

**cdata is placed below the staff by default, and *above can move it above. Add **adata and **bdata which will by default placeaboveandbelow` without the need to add the placement tandem interpretation.

[craigsapp]

Implemented in commit https://github.com/rism-digital/verovio/commit/5fe18a83b07c4bb8f323886576ca3e58aff36af3

Note the order of spines is from close to further from the staff depending on the order of the spines after **kern.

A future enhancement will be to add a label to the start of the **[ab]data that would display "A:" , "B:" etc.