land cover classification notebook -- switches, data downloads, metadata

[gaelforget]

Some changes to

• allow using either the RGB or the larger data set
• download data automatically if needed (to tempdir)
• set the degree of optimization (or 0 to speed things up)
• better inform, including about data set
• fix the accuracy function (?)
• tweak parameters to improve convergence and reduce wait time

[gaelforget]

@rafaqz @Alexander-Barth , please take a look if you get a chance

[gaelforget]

I just reran notebook (on this config) with up to date dependencies.

Any objection to merging?