Color code for OMEGAlpes graphic representation

Before updating OMEGalpes documentation regarding its graphic documentation with its current version (commit 61c9d79c) , some questions and thoughts:

• What color for:
  • Gas : green that is more common for gas instead of the current yellow
  • Electriity : blue that is also more standard, but currently for cold
  • conversion_unit : black empty rectangle rather than green full one.
  • Differentiating hot and cold or keeping thermal energy type. Question of the thermal dissipation units where hot and cold would not be able to mix.