simple connections (cell type collapse) improvements

[stephenplaza]

• add a checkbox on the top to examine only 'Traced' neurons (that should be the default). When only showing the traced neurons, there should be a percent of connections that are traced next to the checkbox
• when collapsing neurons, show the average connection weight for the neuron class ... sort by this value, show a the number of neurons that are in this average in parenthesis
• the '+' does not work in cell type collapse mode ... show the list of body ids that make up this cell type if expanded
• (nice to have) create a slider bar for the cell type that provides a threshold for to group neurons of the same cell type. If the slider is at one extreme, there is no grouping (which is the default when clicking on the plugin now). The other extreme is the current cell type collapse option. Somewhere in between can be different thresholds for grouping cell types together. For instance, one could group cell types only if the connection strength is within 2x.