Get rid of implicit broadcasting for read procs taking indices

[Vindaar]

Currently the procs to read a subset from a dataset instead of the whole will broadcast the seq[int] indices given to all dimensions of an N dimensional dataset. This was introduced as a convenience tool.

However, looking back at this after a while I realize this is just confusing and not intuitive.

Instead we should either use as a default an interpretation of the indices of the flattened data or introduce a better way to read indices for multidimensional datasets that allows for broadcasting via some symbol. Some inspiration could be taken from arraymancer's accessor [] macro.