Be able to gain access to the "File" API through "Dir" API

[manoss96]

All classes within the Dir API should be able to provide instant access to a file through the File API, for example:

from fluke.storage import LocalDir

# Reference a directory through the "Dir" API.
dir = LocalDir(dir='path/to/dir')

# Gain access to the "File" API through a "Dir" method, e.g. ``get_file``.
file = dir.get_file(path='path/to/file')

# Interact with a file through the "File" API.
size = file.get_size()

In the case of remote directories/files, all File instances generated by such method should have the following properties:

1. All files should share the same connection with the directory that generated them.

It should not be required for the user to explicitly close each generated file instance. Closing the directory should be enough. For example:

from fluke.storage import RemoteDir

# Reference remote directory.
dir = RemoteDir(auth=auth, path='/path/to/remote/dir/')

# Reference remote file.
file = dir.get_file(path='file.txt')

# Close connection.
dir.close()

# This should throw an error since connection has been closed.
size = file.get_size()

Similarly when using a context manager:

from fluke.storage import RemoteDir

auth = RemoteAuth.from_password(**credentials)

with RemoteDir(auth=auth, path='/path/to/remote/dir/') as dir:
   file = dir.get_file(path='file.txt')

# This should throw an error since connection has been closed.
size = file.get_size()

2. A "File" instance and the "Dir" instance that generated it should have access to the same metadata.

from fluke.storage import RemoteDir

# Reference remote directory.
dir = RemoteDir(auth=auth, path='/path/to/remote/dir/')

# Reference remote files.
file = dir.get_file(path='file.txt')

# Set metadata through "file"
file.set_metadata({'name': 'file.txt'})

# Access metadata through "dir".
metadata = dir.get_metadata('file.txt') # equals {'name': 'file.txt'}

3. A "File" instance and the "Dir" instance that generated it should have access to the same cache.

from fluke.storage import RemoteDir

# Reference directory and enable caching.
with RemoteDir(auth=auth, path='/path/to/remote/dir/', cache=True) as dir:
   # Get size of directory.
   dir_size = dir.get_size()
   # Reference a file.
   file = dir.get_file(path='file.txt')
   # File size should be fetched from the cache.
   file_size = file.get_size()