Travel expenses (2 points)
==========================

A company wants to analyse the travel expenses of its employees and
has obtained a :class:`list` with the travel expenses for each
of them.  Each item of this list corresponds to the travel expenses of
a month.

Write the **modifier** function :py:func:`expenses` that takes a list,
`te`, as described, and **modifies** it in such a way that each
element accumulates all the previous amounts, i. e., the first element
does not change and the next ones will have the value: :math:`te_i =
\sum_{n=0}^{i} te_n, 1\leq i<len(te)`. Moreover this list must include a
new item at the end with the average of the expenses.

Save this function in file ``expenses.py``.

Examples:

.. literalinclude:: test-expenses.txt
      :language: python
      :lines: 3-15

.. note::

   More tests are provided in file :download:`test-expenses.txt`.