make sensirion driver more robust

switch to pressure control when sensirion returns bad values

tcl/drivers/sensirion.tcl

@@ -61,6 +61,13 @@ proc stdConfig::sensirion {} {
 
 proc sensirion::updateFlow {} {
   lassign [sct result] flow stddev
+  if {$flow == 0 && $stddev == 0} {
+    if {[hgetpropval /nvflow flowsource] eq "flowmeter"} {
+       hsetprop /nvflow flowsource flowpress
+       hepump eco 0
+       error "invalid flow -> switched to pressure"
+    }
+  }
   sct update $flow
   updateval [sct]/stddev $stddev
   if {[silent "" hgetpropval [sct]/save saved] eq "" && [silent "" currentCalib] ne ""} {