Add dolfin timers

[finsberg]

As part of running convergence test we need ways to measure the time it takes for different methods.

Here I add dolfin timers to a few of the methods in the solver and model class. When running a program you can now add

import dolfin as d 

d.list_timings(
    d.TimingClear.keep,
    [
        d.TimingType.wall,
    ],
)

at the end, at it will list the time spent in the different methods. We can also save the results to a file using

res = d.timings(
    d.TimingClear.keep,
    [d.TimingType.wall, d.TimingType.user, d.TimingType.system],
)

from pathlib import Path

Path("timings.txt").write_text(res.str(True))

One question is whether we need the stopwatch class. Thoughts?

[emmetfrancis]

This looks good to me. Are there advantages to using the dolfin timer that we don't have from the previously defined stopwatch class? If so, I am on board with changing the other stopwatches to dolfin timers wherever we might want to keep them.

[finsberg]

This looks good to me. Are there advantages to using the dolfin timer that we don't have from the previously defined stopwatch class? If so, I am on board with changing the other stopwatches to dolfin timers wherever we might want to keep them.

I think that if we can achieve the same with the dolfin timers then we should use that. The stopwatches in smart are really nice but in my opinion it might make more sense to move them out to a separate package since they don't really belong in smart. Also having our own implementation of stopwatches also means that there is one more thing we need to make sure is working as expected.

[jorgensd]

Feel free to merge.
You could use decorator for some of these https://bitbucket.org/fenics-project/dolfin/src/fd662efc1c0ddefa341c1dac753f717f6b6292a8/python/dolfin/common/timer.py?at=master#lines-65