[WIP] Import fix

[xivh]

I have fixed the import error caused by an existing properties.calc.json as described in #320. I fixed the segfault for empty force in #313. For an empty value field like this:

    "atom_properties": {
        "force": {
          "value": []
        }
    }

the import will fail, but there is now an error message instead of a segfault. If value is missing there will also be an error message and the import will fail. The import is already successful if there is no force field, but now I have hardcoded force to be always optional, so the import will work for the above JSON. This seems a bit risky - it would probably be better to pass a list of optional properties.

I also updated some documentation:

• import_properties is false by default
• changed the help string from additional_files to copy_additional_files
• copy_additional_files requires copy_structure_files
• copy_structure_files implies import_properties because it copies properties.calc.json

Two things that I did not address are that the copy_additional_files option prints every copied file to stdout, which is a lot of printing, and the documentation for cp_files says that it returns whether or not files were copied, but I don't think that it does.

[darjaved]

the fix contains files with the extension "cc" , but the original file are with the extension of "hh".