Swb

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- Added the constraint Dr ≤ TAW.

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- Added simple model to get effective precipitation from total
precipitation.

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- Fixed crash when calculate_crop_evapotranspiration received a time
series whose timestamps did not end in 00:00.

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- Fixed crop evapotranspiration model, which could only run for the
entire period from planting to harvest; now it can run for part of the
period.

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- Changed the way deep percolation is calculated; now uses ``theta -
theta_fc`` rather than ``theta_s - theta_fc``.

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- Fixed swb errors with deep percolation.