PlatformIO compatible slave

[vermut]

Opening PR according to request in vermut@569ee74#commitcomment-57467186

@angeloc how to do want me to proceed? my code differs much from yours.

[angeloc]

@vermut rebase on master, apply your patches on top while reworking them, when you have something working please let me know so I can test it!
Greatly appreciated!

[rfvermut]

Do want me to delete other folders as I did in my repo? Because I needed to do that so PlatformIO can use that repo as a source.

On that case you will need to create separate repos for SimpleModbusMaster and SimpleModbusSlaveSoftwareSerial

[rfvermut]

See https://docs.platformio.org/en/latest/core/userguide/lib/cmd_install.html#git for PlatformIO options

[OptimalControls]

Dear people, Is this library compatible with Arduino FIO?

[rfvermut]

I have no idea. Chip seems to be the same, so I would say yes.

[OptimalControls]

I have no idea. Chip seems to be the same, so I would say yes.

I have tested V10 with fio, but when we connected 4 or more devices, Modbus RTU started to give Comm fail.