Update to user manual

[oscararenas12]

User Manual updates:
Section 4.2.3.1 Saccade Velocity Fixes (page 17) says:

• "Mean saccade velocity is the average speed during a saccade, calculated by summing up all velocities during a saccade and dividing by the total amount of points."
  Update made:
• " Mean saccade velocity is the average speed during a saccade, calculated by summing up all velocities during a saccade and dividing by the total number of velocity measurements (which is one less than the total number of points, since n points produce n-1 velocities between consecutive points), excluding any measurements that exceed the velocity threshold. If no valid velocities remain, NaN is returned.”
  Note: This update refers to both handling of 0.0 for peak velocities and mean velocity