Feat/improvements

[andy9a9]

• replace if with elif where possible
  • if the first condition is true the next one can not be, so it doesn't make any sense to use the next if condition. This has a bunch of improvements on performance.
• add generator_version attribute into symbol and footprint
  • Kicad v8 has this attribute defined to "8.0". So once reading/writing this kind of file the value shouldn't disappear.