Merge
Memory optimisation.

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Only cells with numerical types can be formatted as dates.

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Resolves #264

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Merge
Merge in changes previouosly made in default.

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Merge

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Break out of parsing loop as soon as row/column boundaries are reached.

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Allow cells to be accessed individually.

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Resolves #264 - fixes font settings for conditional formatting.

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Only ever need one copy of an empty cell and the coordinates should be irrelevant.

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Add some utility metaclasses.

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Always iterate.

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Move IterableWorksheet into Worksheet package.

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rename : openpyxl/reader/iter_worksheet.py => openpyxl/worksheet/iter_worksheet.py
Update docs.

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Refactor into separate module.

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Cleanup imports and Python 3 compatibility

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Cleanup imports.

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Rename RawCell to ReadOnly and add some checks to stop value or style being changed.

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Move cell tests to package.

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rename : openpyxl/tests/test_cell.py => openpyxl/cell/tests/test_cell.py