Add pca-beta process

[marcellevstek]

REID-2321

Checklist

• Update CHANGELOG.rst for each commit separately:
  • Pay attention to write entries under the "Unreleased" section.
  • Mark all breaking changes as "BACKWARD INCOMPATIBLE:" and put them
    before non-breaking changes.
  • If a commit modifies a feature listed under "Unreleased" section,
    it might be sufficient to modify the existing CHANGELOG entry from previous
    commit(s).
• Bump the process version:
  • MAJOR version (first number): Backward incompatible changes (changes
    that break the api/interface). Examples: renaming the input/output, adding
    mandatory input, removing input/output...
  • MINOR version (middle number): add functionality or changes in a
    backwards-compatible manner. Examples: add output field, add non-mandatory
    input parameter, use a different tool that produces same results...
  • PATCH version (last number): changes/bug fixes that do not affect
    the api/interface. Examples: typo fix, change/add warning messages...
• All inputs are used in process.
• All output fields have a value assigned to them.

[romunov]

@jkokosar please see my review and give feedback on my thoughts on handling the species and source fields internally.

[jkokosar]

@romunov @marcellevstek the species and source inputs in the original process relate and inform the genes input, which is usually provided by the FE module (based on the genes basket state). Even though appropriate controls are implemented elsewhere, It must not be assumed that the source/species of input genes will always be the same as those of expression objects on the process inputs. It is thus not ok to remove the process inputs and related checks within the process.

[romunov]

Looks good. Squash and rebase, please.