Streamtracer

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New features
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* Added the ability to trace field lines through non-uniformly spaced grids.
To do this pass a list of grid coordinates to the new ``grid_coords`` argument in `-streamtracer.Grid`.

Bug fixes
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* Fixed coordinate values returned by `streamtracer.Grid.xcoords` etc.
Previously the origin coordinate was interpreted negative what it should be.
This doesn't affect any traced streamlines.

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* Fixed the example code listed above.

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* Fixed wheel building to use the oldest supported version of numpy for each major version of python.
This fixes errors like "module compiled against API version 0xe but this version of numpy is 0xd" that were in the 1.1.0 wheels.

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* Fixed handling of steps going out of bounds.
Previously, in the forward direction a single step would be saved out of bounds, but in the backwards direction the streamline ended before going out of bounds.
Now both the forward and negative directions both save a single out of bounds step in each stream line.
* Linux and macOS binary distributions (wheels) are now automatically built and uploaded, meaning you should no longer need a FORTRAN compiler installed locally to use streamtracer.
* Minor performance and memory improvements have been made.

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* Fix compilation issues on MacOS Big Sur.

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* Nothing major, just re-versioning to a stable release number.