Aquacrop

Updated Readme describing model features and background

What's Changed
Thanks to oleksmialyk for the fixes and improvements:
* check_groundwater_table.py now correctly returns NewCond_zGW variable
* Removed incorrectly hardcoded 15mm irrigation for IrrMngt_IrrMethod == 1 in irrigation.py
* root_development.py now correctly returns NewCond_rCor
* User input of bund height is still in metres but this is now converted immediately to mm upon initialisation of fieldManagement for consistency of units in calculations.

**Full Changelog**: https://github.com/aquacropos/aquacrop/compare/v3.0.8...v3.0.9

What's Changed
* Minor update to calculation of GDD growth stages when switching from calendar days.
* New crop parameter 'SwitchGDDType' can be specified as 'mean' (default) or 'median', calculates GDD growth stages from calendar day growth stages, using all simulation years. 'mean' or 'median' refers to how the calculation summarises GDD values for each growth stage across all years.
* New crop specification 'SugarBeetGDD_UK'.

**Full Changelog**: https://github.com/aquacropos/aquacrop/compare/v3.0.7...v3.0.8

Fixed issue with fresh yield calculations.

What's Changed
Minor fix to alleviate dependency issue, change cffi from 1.15.0 to 1.16.0.

What's Changed

Changes to manifest.in, requirements.txt and setup.cfg to allow packaging of future climate .csv files and fixing of new dependency issues.