Add Initial Guess Feature

[Poko18]

Hi,

This pull request implements the --initial-guess feature, inspired by the functionality available in ColabDesign.

Related issue:

• --initial_guess feature #432

Description:

• It enables the handling of .pdb inputs by parsing the pdb file and retrieving chain sequences
• The addition of the --initial-guess flag allows for the provision of initial guess .pdb input either as the main input or via the --initial-guess flag
• Minor adjustments to the Alphafold code have been made to accommodate this new feature