update the nessai interface for v0.7.0

Update and improve the nessai interface

Lots of these changes are motivated by the upcoming interface for ins-nessai and avoiding duplicate code. They also take advantage of new features in 0.7.0.

Note: this MR is NOT required to use v0.7.0 of nessai with bilby

Changes

• Refactor class so that it's more modular (ins-nessai will inherit from Nessai and replace a couple of methods)
• Always disable signal handling in nessai and instead handle it in bilby directly using signal_wrapper (I've updated the relevant tests).
• Use nessai.utils.bilbyutils to get keyword arguments instead of doing it in bilby.
• Require nessai>=0.7.0
• Add options to bilby to control nessai logging and plotting independently of the value set in bilby (no more pages and pages of logs!).

To-Do

• More tests of signal handling (I want to sure it still works on the clusters)
• Need !1159 (merged) to be merged and then rebase
• Check for changes in the documentation
• Decide if we want to configure nessai to log to sys.stdout instead of sys.stderr which should be possible as of v0.7.0 but will require a small change to bilby.