Update io.py

[n3011]

Fixes unnecessary error reporting. Mean shape incompatible with input shape

[seanbell]

1. Why is this unnecessary error reporting? The mean image is spatially dependent, so it doesn't make sense to always blindly resize it. I think a safer option to get your feature is to add a parameter to the function (default False) that allows for resizing if the shapes don't match.
2. If your mean image a constant value (which is a reasonable input) then this will introduce a NaN since m_min = m_max.

[seanbell]

The added lines 258-265 are indented by 2 spaces too many. This is a syntax error I believe.

The old code is left there commented out.

[Noiredd]

Fixed in #5719