Numbagg

Numbagg will now compile with`mode="cpu"` if it detects that it's being run in a `ThreadPoolExecutor`. Previously, the default `mode="parallel"` could cause numba to abort the python program within that context.

Note that running in a multi-_process_ context retains `mode="parallel"`, so the new behavior should only be slower in infrequent cases, such as a local dask multi-threaded executor.

I'm not completely confident this is the globally optimal solution, so this may evolve. https://github.com/numba/numba/issues/9288 has more context.

0.6.2 allows grouping functions to take a wider range of `int` types as labels. Thanks to dcherian for the contribution.

- Enables `parallel` mode in most functions. This radically improves performance in multi-core systems on multi-dimensional arrays (see benchmarks for details)
- Allows passing an array of alphas in the `moving_exp` functions, which lets us decay values by different amounts
- Improves `nanquantile`'s compatibility with various `axis` values
- Extends benchmarks to different shapes, adds `bottleneck` as a comparison

- Add `ffill` & `bfill`, at ~2.7x pandas' performance
- Add standard moving window functions — `move_corr`, `move_cov`, `move_std`, `move_sum`, `move_var`, in addition to the existing `move_mean`. These have 3.5-20x pandas' performance.
- New benchmarks using `pytest-benchmark`. This includes a script which makes a nice output which we've added to the readme. It currently only covers the `moving` and `moving_exp` functions.

- Add a `nanquantile` function; approximately 4x faster than `np.nanquantile` when over 2 dimensions. It's slightly slower than `np.quantile` and pandas' `.quantile`
- Ensure we don't produce `inf` values for some exponential moving functions. Numerical values remain unchanged.

- Sets `ddof=1` for `std` & `var` functions, mirroring the grouped & `move_exp` functions (but notably different from numpy)
- Adds a `move_exp_nancount` functions, for exponentially weighted moving counts
- Adds `nancount` as an alias for `count`