Add TXT datatype

[alexrudd2]

Add text (TXT) support.

This is tricky and different from the other datatypes because two 8-bit addresses are packed into a single 16-bit register. For some strange reason it's little-endian.

For example, consider the following state:

address	register	value
TXT1	436865 (LSB)	A
TXT2	436865 (MSB)	B
TXT3	436866 (LSB)	C
TXT4	436866 (MSB)	D

In memory, or using a "naive" Modbus client, this will appear as BADC.

To read TXT1, the whole 16-bit holding register [4]36865 must be read and the first 8 bits (MSB) discarded.
To read TXT2, the whole 16-bit holding register [4]36865 must be read and the last 8 bits (LSB) discarded.

To write TXT1:

• First read TXT2 and save its 8 bit value
• Then pack the 8 bits of TXT2 into the MSB of a 16-bit value, and the 8 bits of TXT1 into the LSB
• Then write the combined 16 bits into holding register [4]36865

To write TXT2:

• First read TXT1 and save its 8 bit value
• Then pack the 8 bits of TXT2 into the MSB of a 16-bit value, and the 8 bits of TXT1 into the LSB
• Then write the combined 16 bits into holding register [4]36865

[alexrudd2]

@cperry-adcTAG