Fix IndexError: Make cb_label right length

[alxthor]

I think cb_label needs to be as long as number of plot panels otherwise one gets list index out of range for cb_label[cb_plt_idx[ii_valid]-1]

Maybe something similar needs to be done with streaml_dat in the line above?
I just tested this for my case:

I noticed the problem when using the hslice template to create a plot with one input path and one reference path. In the template cb_label is set to None, so cb_label would be later set to [None] which is a problem if the second plot panel tries to do_cbar and access the second element of cb_label.