String record handling data truncation

[AlexanderWells-diamond]

When creating a string record, if you set an initial_value that is longer than 39 characters the data is silently truncated. It would be useful if there was at least a warning/error of some kind emitted when this occurs.

In this example note that the trailing "!" of the string is missing:

# Import the basic framework components.
from softioc import softioc, builder, asyncio_dispatcher

# Create an asyncio dispatcher, the event loop is now running
dispatcher = asyncio_dispatcher.AsyncioDispatcher()

# Set the record prefix
builder.SetDeviceName("PREFIX")

# Create some records
bo1 = builder.stringOut('SO', initial_value="hello this is long sentence of 39 chars!")

# Boilerplate get the IOC started
builder.LoadDatabase()
softioc.iocInit(dispatcher)

# Finally leave the IOC running with an interactive shell.
softioc.interactive_ioc(globals())

Getting the value:

[...]$ caget PREFIX:SO
PREFIX:SO                      hello this is long sentence of 39 chars

[AlexanderWells-diamond]

Note that I have already added a tiny amount of logging to the AsyncioDispatcher https://github.com/dls-controls/pythonSoftIOC/blob/master/softioc/asyncio_dispatcher.py#L37

If the warnings module is used for user notification instead then this one should use the same mechanism.

[AlexanderWells-diamond]

This ended up being mostly fixed in #60, then fully fixed in 53f107d, with a test added in 5f14ca3 as part of #79