Resolve "Conversion function not applied to posterior"

Closes #613 (closed).

Changes

• Change interface for nessai and cpnest so save result.samples instead of result.posterior
• Modified the existing integration test for the samplers to check that the conversion function is applied for all of them.
• Add pytest.importorskip to all tests in sampler_run_test.py so tests can be run locally without requiring an environment with all of the samplers installed