Save reference values that are modified

[aidanjungo]

For now only coefficients could be modified.
Reference value (lenght, area and point of ref) should also be modifiable, but the following things should be checked:

• What is the best way to do it?
• How/when apply this modification to coefficients?
• Should we be able to modify other value through cpacspy?

[aidanjungo]

Maybe also the MAC (mean aerodynamic chord) should be calculated, probably using the TiGL value.