Load of nimrod files with multiple fields and period of interest

Signed-off-by: Andrew Munslow <munslowa@gmail.com>
Update post PR review

Signed-off-by: Andrew Munslow <munslowa@gmail.com>
Test result for period of interest test

Signed-off-by: Andrew Munslow <munslowa@gmail.com>
Use updated version of test data

old test files removed. extra newline added

test_osgb_to_latlon now uses the first index of the time dimension for the test

unused imports removed