Implement forced file cache reading and skipping memory cache

This MR implements two new options into Transformation that determine whether the memory cache is used (at all) and whether the result should always be loaded from a cache file:

memory_cache: true   # new option: if false, will not store the computation result, leading to re-computation or forced cache lookup
file_cache:
  read:
    enabled: true
    always: false    # new option: if true, will always read from memory, even if the result was just computed.

Can this MR be accepted?

• Implementation ready
• Tests added or adjusted
• Documentation extended or updated

• Code quality
  • Changes follow coding guidelines
  • Checked code coverage on new and adjusted code

• Ready for merging
  • Pipeline passes without warnings
  • History cleaned-up or squash option set
  • Changelog entry added
  • Version number bumped

Related issues

Closes #337 (closed)