APU Fixes into Dev Branch

[tsalvo]

This PR aims to improve compatibility by fixing some issues with the Audio Processing Unit (APU):

• for APU channels, don't update length counter value if channel is disabled.
• fix some issues with the way APU frameIRQ was set.
• Clear DMC interrupt flag on write to 0x4015.
• Update APU reset logic

changes are based on nesdev.org documentation

[tsalvo]

regression found in TMNT3 where a descending auto tone occurs followed by a crash