define coloramp for for variables

closes #45 (closed)

• new dataclass VariableCustomColorRamp for color ramp creation with custom parameters:

  • color ramp name and inversion
  • is color ramp continous or by interval
  • is color ramp discrete or linear
  • step below and above ten for color ramp steps definition
  • option for transparent minimum value
  • define list for fixed value and their labels

• define VariableCustomColorRamp for each variable

⚠ there is an issue on QGIS with legend after mesh layer load. The legend is not updated for all variables. User must open layer property dialog and apply symbology.