Metview

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- added support for reflected operators on Fieldsets, e.g. "2 * Fieldset"
- done for addition, subtraction, multiplication, division and power

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- added ml_to_hl() function

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- export the Request class
- fixed memory leak when returning a list of items
- allow bool-typed numpy arrays as input
- fixed issue where the Fieldset iterator could fail if used multiple times

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- Metview startup timeout configurable via environment variable METVIEW_PYTHON_START_TIMEOUT (in seconds)
- Metview startup timeout default set to 8 seconds in case of busy systems
- added integral() function
- fixed memory leak when exporting vectors as numpy arrays

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- added equality (``==``) and non-equality (``!=``) operators for Fieldset and Geopoints objects, e.g. ``same = (a == b)`` will produce a new Fieldset with 1s where the values are the same, and 0s elsewhere.
- added new thermodynamic, gradient and utility functions: 'thermo_data_info', 'thermo_parcel_path', 'thermo_parcel_area', 'xy_curve', 'potential_temperature', 'temperature_from_potential_temperature', 'saturation_mixing_ratio', 'mixing_ratio', 'vapour_pressure', 'saturation_vapour_pressure', 'lifted_condensation_level', 'divergence', 'vorticity', 'laplacian', 'geostrophic_wind_pl', 'geostrophic_wind_ml'
- improved conversion from geopoints to pandas dataframe to cope with new NCOLS subformat
- make conversion from Fieldset to xarray dataset compatible with latest versions of cfgrib

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- code cleanup so that tox and pyflakes pass the tests