Implement proper ReconstructSome for leopard

[Ismail Khoffi]

Currently, ReconstructSome for leopard just calls ReconstructData:

reedsolomon/leopard.go

Lines 335 to 337 in 674f18b

 func (r *leopardFF16) ReconstructSome(shards [][]byte, required []bool) error { 
 	return r.ReconstructData(shards) 
 } 

For other implementations, the user can actually specify the required shards to be reconstructed.

I'm advocating to implement the same behaviour for leopard as well.