Wrapped results of SlurmEnvironment.__Slurm_Command in env.Precious
Set all_precious option (default: F) on slurm env initialization; precious behaviour can also be specified in commands
- prevent import error when SCons.Script is unavailable
- add fileutils.check_digest, fileutils.write_digest
increment version
Merge branch '9-all-precious' of https://github.com/metasoarous/bioscons into metasoarous-9-all-precious
update changelog for v0.6
Merge branch 'metasoarous-9-all-precious'
mention Precious feature in changelog