Improvements to InflowWind - Scaling ramp and offset

[ebranlard]

Is your feature request related to a problem? Please describe.

• Starting the simulations with a full wind speed can result in important, undesired transients (since we don't have a static solve and no ways to define the initial blade deflection of BeamDyn).
• Figuring out the offsets of the turbulence boxes can be a difficult and confusing task for new users. In particular, periodic and nonperiodic BTS boxes are placed at different locations at t=0 (see TurbSim manual page 10). (For now, users should preferably use Mann Boxes instead, as there placement is more systematic and tunable using the xoffset parameter.)

Describe the solution you'd like

• Introduce a wind ramp to linearly scale the wind speed. For instance, with two parameters: duration of the ramp and scaling factor at t=0 (scaling factor between 0 and 1).
• Remove the logic behind the placement of periodic and non-periodic boxes. Always place them the same
• Introduce a time-offset (or space offset) effective for Turbulence boxes.