Finitediff

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- Improved setup.py

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- git-archive generated zip now pip-installable

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- Introduced ``finitediff::generate_weights_optim`` which is a pre-sorting
wrapper around ``finitediff::generate_weights`` (tests indicate approx. 1 extra significant figure)
- Introduced ``finitediff::calculate_weights_optim`` which is a pre-sorting
wrapper around ``finitediff::calculate_weights`` (tests indicate approx. 1 extra significant figure)

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- Refactored ``finitediff::generate_weights`` (part of official API from v0.3.0)
- Deprecated ``finitediff::populate_weights``
- Introduced ``finitediff::calculate_weights`` (to replace populate_weights)

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- Fixes to Python distribution (include C++ header)

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- C++11 function ``generate_weights`` provisionally provided