Modifications for compatability with agage-air-archive

[qq23840]

A couple of fixes to make smoother integration with https://github.com/qq23840/agage-air-archive

• Make 'resample' an optional argument in the various run... functions, which defaults to True but can be set to false for air archive/flask data
• Make a 'mode' argument for the plotting in visualise.py and widgets.py, which defaults to line but can be set to 'markers' for scatter plots
• Adds a second option of engine for opening netCDFs, which gives a bit more flexibility
• bug fix for ds.drop_duplicates (may have been my xarray version, but should now work everywhere)

[qq23840]

Looks good to me. Visualisation notebook still doesn't like the datasets with only one measurement (because of the thinning function applied to a single value) but plotting a single value is a bit meaningless anyway...