AP_Baro: slow down the LPF for slewing to a new GND_ALT_OFFSET

Instead of a couple seconds, make it about 15sec. This makes TECS happy by not glitching the height demand as much. When applied too fast the TECS height demand causes a large single oscillation as it chases the filter lag.

libraries/AP_Baro/AP_Baro.cpp

@@ -346,10 +346,10 @@ void AP_Baro::init(void)
  */
 void AP_Baro::update(void)
 {
-    if (fabsf(_alt_offset - _alt_offset_active) > 0.1f) {
-        // if there's more than 10cm difference then slowly slew to it via LPF.
-        // The EKF does not like step inputs so this keeps it happy
-        _alt_offset_active = (0.9f*_alt_offset_active) + (0.1f*_alt_offset);
+    if (fabsf(_alt_offset - _alt_offset_active) > 0.01f) {
+        // If there's more than 1cm difference then slowly slew to it via LPF.
+        // The EKF does not like step inputs so this keeps it happy.
+        _alt_offset_active = (0.95f*_alt_offset_active) + (0.05f*_alt_offset);
     } else {
         _alt_offset_active = _alt_offset;
     }