Calc accessibility

[mgiulini]

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Closes #818 by adding calc_accessibility to the restraints-related subcommands. Now the accessibility analysis can be run within haddock3 with

haddock3-restraints calc_accessibility file.pdb

using the --export_to_actpass option the exposed residues can be authomatically exported (as passive) to an .actpass file of this form, which can then be used to generate a tbl file with haddock3-restraints active_passive_to_ambig