Use pymodbus decoding

[kodonnell]

This is a much cleaner way of doing decoding to Int16s etc. Use this as less maintenance and simpler code.

Need to think a bit ... may be able to get rid of Field._from_registers and Field._to_registers now since they'll largely just be e.g. decoder.decode_16bit_int etc. May be able to just add a read_contiguous_fields method to the modbus client which just does all the nice things for you, including creating FieldValues. Anyway, worth thinking through, especially if it makes the code internals nicer.