implement synthetic.trapezoid function

[dkrako]

Description of the problem

The step function is good for most simple purposes but it results just in single step velocity spikes on differentiation.

Description of a solution

Add a trapezoid function which should work similar as the step function but you should be able to define the gradients of the trapezoid with the possibility for different

Minimum acceptance criteria

• [ ] add a trapezoid() to pymovements.synthetic