QModbusDataUnit has inconsistent signedness for params and return values

`valueCount()` returns an unsigned value - fair enough the number of values can never be negative.

However, all the other accessor functions take `index` as an int - signed by default.

This means there will always have to be a messy static_cast somewhere when looping through the values. Even the example code for `readReady()` is not warning free...

Some consistency would be great please, all signed, or all unsigned, and warning free compiles.

TODO:

• change value()/setValue() to use qsizetype
• change valueCount()/setValueCount() to use qsizetype
• adapt example code and documention to use qsizetype