SICS-646: Restore histogram command to hdb tree.

Instruments which don't have a histogram should add
"set no_histmem true"
to the top of their instrument config file before the commands_common
file is sourced.

site_ansto/instrument/config/commands/commands_common.tcl

@@ -5,7 +5,7 @@
 namespace eval commands { }
 
 proc ::commands::ic_initialize {} {
-if {[info procs "histmem" ] != ""} {
+if { ![info exists ::no_histmem] || ($::no_histmem == false) } {
   namespace eval ::histogram {
   set NS [namespace current]
   variable parameters