add docstrings (& examples) for aggregate methods

[gdementen]

We might want to use a template for part of it.

[alixdamman]

Since aggregate functions are also defined outside LArray class, can I do something like https://docs.scipy.org/doc/numpy/reference/generated/numpy.ndarray.sum.html#numpy.ndarray.sum ?

[gdementen]

Sure, but I wonder if doing the opposite (have the full doc in the method and link to it in the function) wouldn't be better in this case. The reason is that I prefer when people use the methods over the functions. The functions are only there so that they can be used in with_total.