Added condition to 'calc_fd_measures' function to prevent crash

[Arritmic]

• Test data: wrist PPG data from an Empatica E4 (WESAD database)

• Batch processing working with input segments of 10 seconds (640 samples to 64Hz)

• The function "calc_fd_measures" crashes when trying to compute the 1-D smoothing spline (interpolated_func = UnivariateSpline(rr_x, rr_list, k=3) when the "rr_list" variable contains less intervals than the degree of the spline.

  rr_x values: [640.625, 1093.75, 1937.5]
  rr_list values: [640.625, 453.125, 843.75]

  Error: dfitpack.error: (m>k) failed for hidden m: fpcurf0:m=3

• This is nothing related to the final window and insufficient input samples. It crashes in the middle of the signal...

• Added the degree parameter as input argument as well.

[Arritmic]

I close the pull request until I solve the issue related to the CI.