ascent singleton reckoning

[cyrush]

advanced cases of ascent require multiple instances of ascent, but there are a few singletons we rely on, for example:

• expression caching and execution
• flow mpi comm

This is fine we we are using multi process, but won't work with threaded use cases

As part of #1355, we should also resolve this.

[cyrush]

Refactor any use of:

flow::Workspace::default_mpi_comm();

To obtain an mpi comm id from the workspace registry instance.