Ensured that pre_plot_hooks are not executed when changing color channel
Ensured that pre_plot_hooks are not executed when changing color channel

Previously changing the color channel (which also happened during
initialization) would execute the pre_plot_hooks. This call now
happens with the update flag set to False ensuring that measurements
are not executed automatically even though the plot_immediately flag
is set to False.

ioam/topographica,ioam/topographica,Tasignotas/topographica_mirror,ioam/topographica,ioam/topographica,mjabri/topographica,Tasignotas/topographica_mirror,Tasignotas/topographica_mirror,mjabri/topographica,mjabri/topographica,Tasignotas/topographica_mirror,ioam/topographica,mjabri/topographica,Tasignotas/topographica_mirror,mjabri/topographica