Fix performance of slow tests

[andrewphilipsmith]

(Note: This issue was initially opened here https://github.com/urbangrammarai/gee_pipeline/issues/17)

A small number of unittests are very slow. This is a disincentive to regularly running tests.

To get a list of the worst performing tests, use $ pytest --durations=0:

====== slowest durations =========
235.44s call     pyveg/tests/test_analyse_gee_data.py::test_analyse_gee_data
52.06s call     pyveg/tests/test_processor_modules.py::test_NetworkCentralityCalculator
32.30s call     pyveg/tests/test_processor_modules.py::test_Sentinel2_image_processor
30.37s call     pyveg/tests/test_download_modules.py::test_veg_downloader_run_sentinel2
4.41s call     pyveg/tests/test_download_modules.py::test_weather_downloader_run_era5
2.48s call     pyveg/tests/test_data_analysis_utils.py::test_create_lat_long_metric_figures
....

These tests should be reviewed.