add automatic unit simplification. closes #49

[ngoldbaum]

This adds automatic simplification of units in the form of a new Unit.simplify() method. It also makes it so that products and quotients of arrays with units will now automatically be simplified.

Caveats:

• I haven't done any performance benchmarking. In principle calling simplify() in the __array_ufunc__ machinery is "free" because it's behind an LRU cache but I need to actually verify that. There's also an extra multiplication on the array that happens if there's any unit cancellation. In practice though most people would be "manually" doing that simplification and therefore incurring the cost of the extra multiplication anyway. All that said I still need to actually verify that the performance impact is at least acceptable.
• My tests are not super extensive and this is an invasive change. It's entirely possible there are untested corner cases.

[jzuhone]

This is a really good change here in the docs.

[chummels]

Big fan of this PR. Thanks for implementing this. Yes, a substantial change, but definitely for the better.

[ngoldbaum]

I've done some basic performance testing and don't see any significant slowdowns, e.g.:

goldbaum at ROUS in ~/Documents/unyt on simplify!
± python -m perf timeit -s "import numpy as np; data1 = np.random.random(1000); data2 = np.random.random(1000); import unyt as u; data1 *= u.s; data2 *= u.Hz" "data1*data2"
.....................
Mean +- std dev: 9.46 us +- 0.54 us

goldbaum at ROUS in ~/Documents/unyt on master!
± python -m perf timeit -s "import numpy as np; data1 = np.random.random(1000); data2 = np.random.random(1000); import unyt as u; data1 *= u.s; data2 *= u.Hz" "data1*data2"
.....................
Mean +- std dev: 9.35 us +- 0.42 us

Going to go ahead and merge. Thank you for looking this over!