WIP: add index to CalcChannel

I would like to add a parameter to compute function in order to send to calc channels their position in a scan.

It helps me to create "theoretical positions" based on index and start and stop position for VSCANNER movements.
ex: scan21(sampy, start=50, end=51, step=0.2, ....) must produce a counters sampy_axis with values:
[ 50.0 50.2 50.4 50.6 50.8 51 ] mainly to plot the fast scan.

I think it can be useful in other situations.

problem: it changes the signature of compute function...

output_channels_data_dict = self.cbk.compute(
	-            sender, {channel.short_name: channel_data}
	+            sender, {channel.short_name: channel_data}, self.index[channel.short_name]
	        )