Better GeneratorMatrix unencode function

The current unencode function for EncoderLinearCodeGeneratorMatrix is taken from Codinglib where an information set is used to unencode. To compute this information set, I, in want of something clever, just applied a randomized approach. This works ok for well-behaving codes but can be exponential for really bad codes.

However, LinearCode already has an information_set function which is well-implemented. This should just be used instead.