Release 1.3.1

Bugfixes and improvements since 1.3.0

Generic

• analogWrite: delete pwm object if duty < 0 (useful on nRF52 since PWMs are limited)
• USBSerial: fix sending more than CDC_MAX_PACKET_SIZE in a single shot
• Tone: allow pitches higher than 500Hz

Portenta

• Port PluggableUSBMSD (allows exposing any mass storage medium over USB)
• WiFi: fix AccessPoint functionlity
• CM4: route Serial object to UART1