Fix converter for ndtiff using multicam

[edyoshikun]

This PR fixes handling ndtiffs written with mutlicam. When using multicam, the datasets have an extra axis that needs to be parsed to reshape the output array.

[edyoshikun]

@ieivanov this is the PR I started trying to fix the convert to have iohub read the data with the additional axis.

[ieivanov]

Thanks Ed - let's see if we can fix this problem at the source. I don't like that in this implementation we need to expand the array dimensions from 5D to 6D. In the meantime, I understand that this is helping you convert the data that's acquired with the intermediate fix we have.