Add idea of plot_group, to plot more than one variable on a chart.

netcdf.py: Misguided attempt to reduce memory usage by appending a
variables values in the concatenated array right after reading them.
I don't think it actually helps, could be reverted.
dim2 is a dictionary, containing a name and data, eventually units.

ncharts.js: Combine variables in a plot_group into a chart, using
successive yAxes if their units are different. Toggling the
display of a trace by clicking on the variable name in the
highcharts legend is a very nice feature.
Add dim2 units