Webgrid

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- Allow columns to be grouped in the UI with an additional header (HTML, XLSX targets) (3a9e64a_)
- Fix bug clearing single filter when sessions are enabled (c14c708_)

.. _3a9e64a: https://github.com/level12/webgrid/commit/3a9e64a
.. _c14c708: https://github.com/level12/webgrid/commit/c14c708

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- fix session persistence for flask (5ed62b8_)

.. _5ed62b8: https://github.com/level12/webgrid/commit/5ed62b8

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- fix issue searching date(time) fields for values that are out of bounds (3d33663_)

.. _3d33663: https://github.com/level12/webgrid/commit/3d33663

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- fix DateFilter/DateTimeFilter default op bugs introduced in 0.2.5 (edeb74e_)

.. _edeb74e: https://github.com/level12/webgrid/commit/edeb74e

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- reduce code complexity (e588630_)
- handle missing filter values for DateFilter an DateTimeFilter (b0e614b_)
- note: this release has some bugs in DateFilter/DateTimeFilter for handling default op

.. _e588630: https://github.com/level12/webgrid/commit/e588630
.. _b0e614b: https://github.com/level12/webgrid/commit/b0e614b

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- split filter key matching for search to a separate method for easier override (95032d2_)
- fix DateTimeFilter between operation when missing second operand (acfc86b_)

.. _95032d2: https://github.com/level12/webgrid/commit/95032d2
.. _acfc86b: https://github.com/level12/webgrid/commit/acfc86b