FolderData should take folder path as constructor argument

[ltalirz]

We discovered during the SINTEF tutorial that it is quite difficult to figure out how to create a FolderData from an existing folder (I figured it out by searching the aiida-core source code, which is not something a regular user can do).

• When creating fd = FolderData in verdi shell and doing fd. + you see lots of methods, but the relevant one ( .put_object_from_tree) was not even visible (you would have to scroll to the end)
• When you just want to create a FolderData to wrap a folder on your file system, "put_object_from_tree" is maybe not what you are thinking...

In my opinion, the FolderData should have a constructor argument folder to which you can pass an absolute path to a folder that will make the FolderData wrap this folder.
I think this would cover the majority of cases with a simple user interface.

Mentioning @sphuber and @giovannipizzi for comment

[sphuber]

The way this works in aiida-core==1.0.0 is that the constructor of Data sub classes can define the keywords they want to accept in the constructor to initialize the node. In this case this would look like:

class FolderData(Data):

    def __init__(self, **kwargs):
        """Construct a new `FolderData` to which any files and folders can be added.

        To add files to a new node use the various repository methods:

            folder = FolderData()
            folder.put_object_from_tree('/absolute/path/to/directory')
            folder.put_object_from_filepath('/absolute/path/to/file.txt')

        Alternatively, in order to simply wrap a directory, the `path` keyword can be used in the constructor:

            folder = FolderData(path='/absolute/path/to/directory')
        """
        path = kwargs.pop('path', None)
        super(FolderData, self).__init__(**kwargs)
        if path:
            self.put_file_from_tree(path)

I can make a PR for this if we are happy with the interface. Since it is a FolderData I now only added option to add a directory. Should we also add one for adding a single file? Of course after construction, one can add as many other things as one wants