MaskWriteRegisterRequest/MaskWriteRegisterResponse don't belong in the file_message module

[idahogray]

I was looking around for these classes and had to look pretty hard to find it in the file_message module. I think they would fit better in the register_write module.

I would be happy to submit a pull request if you agree.

[dhoomakethu]

@idahogray sorry for the late reply, please raise a PR if it is handy

[dhoomakethu]

MaskWriteRegisterRequest/MaskWriteRegisterResponse are now part of register_write_message.py (from 1.3.1)