Continuing to update my terminal code to better emulate the 3174-3178 behavior

While I am waiting for Andrew Kay to make some modifications in his oec 3174 emulator project necessary to support the keyboard on my terminal, I can improve my understanding of the terminal and have fun playing around.

I was expecting to receive my 3179 color terminal yesterday, but Fedex abruptly notified me late in the afternoon that they would deliver today rather than yesterday. I had noticed it was listed as sitting in Tracy and not on a truck all day.  I then turned to the Python code to enhance what I had been doing.

My initial work put out a fixed screen of data and managed the fields based on hardcoded locations. Far better for me to emulate the full functionality with arbitrary screen contents and any number of fields at any locations. Thus I built a class called Buffer that would handle this for me and do write-through to the regen buffer inside the 3178.

It builds a list of field tuples upon request. This is called whenever the 'host' writes to it (or my startup where I set initial screen contents). This is a bit wasteful of processor time but safer than trying to sort out the minimum changes to the attribute list based on each write from the host. 

Having the list of attributes allows me to provide functions that validate whether a field is protected or numeric. Eventually it will allow Insert and Delete key operation which moves data but stops at the end of the current field.