REFAC: Make scene tasks sequential by default

This makes Scene tasks sequential by default. This should be safer and cause no parallel/indeterministic gotcha's. The one con:

• When many simulation objects are used but each is not well parallelized. Then task parallelization provides performance improvements (such as pbd string example). Such cases are very rare, normally models are well enough loop parallelized that it makes no difference if task parallelization is used or not. In the few niche cases where you are using multiple simulation objects that aren't well parallelized you can still switch it on.

Summary: Safer off.