Fix error - assigning cells to double

blockedReactions was initially declared as a double. However, model.rxns(i) is a cell and can't be assigned to a double. Fixes it by declaring it as an empty cell array.

- As pointed out by @andrevo, the metabolite renaming in checkCobraModelUnique is fixed.

- As pointed out by @bheavner, the ambiguities in extractSubNetwork are fixed. Also, fields like rxnNames, rules and metCharge are added to subnetwork.