add timing instrumentation

[orbeckst]

We want to see how much time is spent in various portions of the code. This would store raw time durations for various pieces of code in a dict-like structure and make the data available to Python code.

This would be in addition to logging #107

[orbeckst]

For example, see the Timing class in PMDA https://github.com/MDAnalysis/pmda/blob/e6ac508338b1e6513eab15eadc37e3ec5356cb17/pmda/parallel.py#L33 and how it is used.

[orbeckst]

PMDA used timeit() context manager https://github.com/MDAnalysis/pmda/blob/master/pmda/util.py#L26, eg https://github.com/MDAnalysis/pmda/blob/e6ac508338b1e6513eab15eadc37e3ec5356cb17/pmda/parallel.py#L369