New firmware for the Swissmicros DM15L

[jebedeo]

Hello,

Here is another beta, Thanks for all the bug findings Jean-Charles!

1. display with FIX/SCI/ENG should now be fixed
2. SST not displaying the second line of code when something is in the i/o buffer is now fixed
3. STO/RCL et cetera, which I call settings in my code, can now be canceled by a backspace, without deleting the X register. Since every setting can be cancelled that way, I am not sure what f PREFIX should do. perhaps an extra functionality if there is memory left?
4. DSE/ISG are now implemented. I tried the example from the manual and it works, but I am sure there are bugs somewhere, so I will test some more in the next few days.
5. fixed the overflow bug in -> H and -> HMS that you found, thanks for spotting it!