Force encoding in dmtx

[uidzip]

Since swapping to dmtx, Barby::DataMatrix.new now autodetects the encoding to use and cannot be overridden. Semacode did not do numeric encoding so this wasn't a problem, but we have mail inserter equipment, 100s of $thousands, that cannot read numeric encoding datamatrix.

I will fork and do a PR unless someone beats me to it, but we need to be able to tell dmtx to use ascii encoding even if the data is numeric, through barby.

[uidzip]

That actually isn't the problem, both are using ascii encoding. The more I dig into this the weirder it gets. Semacode used someone else's encoder function which had a one-off bug, and rather than fix it the semacode author just appended a space to the data, artificially increasing the symbol size. It also uses a lookup table to determine symbol size rather than calculate it efficiently the way dmtx does.

What I actually need is a way to force dmtx to artificially use a less efficient symbol size than is required for the data. The inserters will read 18x18 but they will not read 14x14. I don't know why, I didn't build them and I can't force them to support 14x14. So I may have to do a PR on both barby and dmtx.