api: Public SetCell

[Wondertan]

Currently, only a private setCell method is used internally to set repaired cells, while we can enable it publicly so that API users can fill cells/shares on the fly during the repairing process. Currently, a user has to alter the flattened data square slice imported with importing helper, which requires a relatively complicated formula) to calculate cell position. Instead, users would benefit from public coordinates based SetCell.

Additionally, SetCell should allow setting values only for the nil cells to prevent breaking the integrity of the imported square verified by the imported and cached roots.. This also means that public SetCell won't need to reset roots like its private bro setCell does.