Indexable type generates linter warning with numpy arrays

[afedynitch]

When passing the covariance as numpy array, the Indexable type generates linter warnings of the type:

Argument of type "NDArray[float64]" cannot be assigned to parameter "cov" of type "float | Indexable[float] | Indexable[Indexable[float]]" in function "propagate"
  Type "NDArray[float64]" cannot be assigned to type "float | Indexable[float] | Indexable[Indexable[float]]"
    "NDArray[float64]" is incompatible with "float"
    "NDArray[float64]" is incompatible with "Indexable[float]"
    "NDArray[float64]" is incompatible with "Indexable[Indexable[float]]"Pylance[reportGeneralTypeIssues](https://github.com/microsoft/pyright/blob/main/docs/configuration.md#reportGeneralTypeIssues)