Update Measure and EchoMeasure to add new measures and standardize existing ones

[nathanpainchaud]

• Standardize the units of values returned (and/or the documentation of the return value) for previous methods: surface_area, structure_area_split_by_endo_center_line, lv_base_width and lv_length. The units were chosen as ones values in the literature are typically reported in;
• Add new EchoMeasure methods + test of the new measures on synthetic examples:
  • myo_thickness
  • curvature