Conversion improvements

[janimm]

Let me propose some improvements into the conversion.

• units="None" can be omitted from the output
• units="deg C", units="deg F" or units="deg K" should be units="C", units="F", units="K"
• signed="false" can be omitted from the output
• rangeMin="None" and rangeMax="None" can be omitted from the output
• endianess="little" can be omitted from the output, especially from value tags when endianess is already set into owning frame.
• the startbit and bitcount attributes should be replaced by offset and length attributes when they fit into 8 bit boundaries. This gives some performance benefits when parsing incoming data. For example:
  • startbit="0" bitcount="8" -> replaced by offset="0" length="1"
  • startbit="16" bitcount="16" -> replaced by offset="2" length="2"