Powerbox

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**Bugfixes**

- Fix error when doing cross-power [Issue 5].

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**Features**

- New ``left_edge`` argument in fft/ifft which gives the ability to specify where
the x- (or k-) co-ordinates are, in order to set appropriate phase information.
NOTE: this changes the default behaviour of the function. While the forward and
inverse transforms remain inverses by default, the phases are interpreted as
having zero at the centre (for both transforms). See the phasing tutorial for
more information.
- Fixed transpose issue caused by default behavior of ``numpy.meshgrid``, which
led to broken correspondence between discrete sample of field and original
field. See [Issue 15].

**Bugfixes**

- Make warning about pyFFTW slightly less obnoxious.

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**Enhancements**

- Added ability to use weights on k-modes in ``get_power``.

**Bugfixes**

- Fixed bug on using ``ignore_zero_mode`` introduced in v0.5.6
- Added tests for ``ignore_zero_mode and ``k_weights``

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**Enhancements**

- Added ``ignore_zero_mode`` parameter to ``get_power``.

**Bugfixes**

- Removed redundant ``seed`` parameter from ``create_discrete_sample()``.

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**Bugfixes**

- log_bins wasn't being passed through to angular_average correctly.

**Enhancements**

- ``angular_average()`` no longer requires coords to be passed as box of magnitudes.
- improved docs.
- fixed source divide by zero warning in PowerBox()

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**Enhancements**

- Added ability to do angular averaging in log-space bins
- When not all radial bins have co-ordinates in them, a more reasonable warning message is emitted.
- Removed redundant bincount call when only summing, not averaging (angularly).

**Bugfixes**

- Now properly deals with co-ordinates outside the bin range in angular_average (will only make a difference when bins
is passed as a vector). Note that this has meant that by default the highest-valued co-ordinate in the box will *not*
contribute to any bins any more.
- Fixed a bunch of tests in test_power which were using the wrong power index!

**Internals**

- Re-factored getting radial bins into _getbins() function.