Add docstrings for FileSource class and its methods

[samsammurphy]

The FileSource class should probably have a docstring. As should it's methods, for example head().

In my case I was going through the notebook documentation. The first example in the first notebook is..

import earthkit.data
fs = earthkit.data.from_source("file", "temp_10.bufr")

The markdown comment immediately after that says "We can call head(), tail() and ls() to see metadata from the header section of the BUFR messages: "

My instinct was to run the jupyter shortcut for help...

fs.head?

but the output was

Signature: fs.head(n=5, **kwargs)
Docstring: <no docstring>
File:      ~/code/earthkit-data/earthkit/data/sources/file.py
Type:      method

i.e. no docstring describing the head() method

[sandorkertesz]

This is related to #40