Adding action to bundle PDFs

Created by: siranipour

An action that closes #1331 (closed). You specify the base pdf as pdf and the alphas pdfs for which you want to add the replica 0s from with pdfs.

Example runcard:

pdf: NNPDF31_nnlo_as_0118_DISonly

pdfs:
  - NNPDF31_nnlo_as_0118
  - NNPDF40_nnlo_as_0118


actions_:
  - bundle_pdfs

Edit: you can find the bundled PDF set inside the output folder. Note it won't overwrite the bundled PDF if it already exists