Add support for focused sampling with SOMD2

[lohedges]

This PR modifies the SOMD2 parquet extraction code to always use the lambda values in the table, minus those used for gradients, rather than the lambda_array metadata, which has now been deprecated. This allows sampling of different lambda windows over time for restart simulations, e.g. if you want to focus much more sampling on a particular region of lambda space.

• I confirm that I have merged the latest version of devel into this branch before issuing this pull request (e.g. by running git pull origin devel): [y]
• I confirm that I have added a test for any new functionality in this pull request: [y]
• I confirm that I have added documentation (e.g. a new tutorial page or detailed guide) for any new functionality in this pull request: [y]
• I confirm that I have permission to release this code under the GPL3 license: [y]

[mb2055]

All good 👍