add deep sleep mode

esp32_readout/display.cpp

@@ -34,4 +34,9 @@ void Display::print_weight(double val) {
   this->_display->setCursor(12, this->mid_x);
   this->_display->printf("%.2f g", val);
   this->_display->refresh();
+}
+
+void Display::clear() {
+  this->_display->clearDisplay();
+  this->_display->refresh();
 }