Add the ability to set CTD

[alexrudd2]

CTD registers are read/writeable in the ClickPLC, but the current code only has get_ctd().

• Extend to set_ctd()
• Ensure the correct warnings are in place (e.g. trying to set CTD251 should give an error message
• Write a new unit test for the roundtrip (probably test_dd_roundtrip() is a good basis)

Also, either here or in a separate issue:

• Extend get_ctd() to reply with a single value instead of a dictionary to match other register types
• Fix the test that breaks when doing so

[alexrudd2]

@andygarcha-adc why don't you try this one? You are very close to the code for it so it should be good practice picking-and-choosing. Your current commit for adding this feature has multiple things in one commit, so it's also good practice to split that commit in two.