Fix: make sure that precip_models variable has the correct length

Fix to ensure that the `_check_inputs` function in `/blending/steps.py` looks for the correct `precip_models` length when a list of timesteps instead of an integer is provided.

pysteps/blending/steps.py

@@ -1808,7 +1808,7 @@ def _check_inputs(
  if isinstance(timesteps, list) and not sorted(timesteps) == timesteps:
  raise ValueError("timesteps is not in ascending order")
  if isinstance(timesteps, list):
- if precip_models.shape[1] != len(timesteps) + 1:
+ if precip_models.shape[1] != timesteps[-1] + 1:
  raise ValueError(
  "precip_models does not contain sufficient lead times for this forecast"
  )