Make erasure matrix type not optional choose automatically

Remove option of providing Technique and handling errors based on that
choose a matrix type automatically based on number of data blocks.

INTEL recommends on using cauchy for consistent invertible matrices,
while vandermonde is faster we should default to cauchy for large
data blocks.

pkg/erasure/doc.go

@@ -39,10 +39,9 @@
 //
 // But here are the few points to keep in mind
 //
-//  Techniques:
+//  Matrix Type:
 //     - Vandermonde is most commonly used method for choosing coefficients in erasure
 //       encoding but does not guarantee invertable for every sub matrix.
-//       Users may want to adjust for k > 5. (k is data blocks)
 //     - Whereas Cauchy is our recommended method for choosing coefficients in erasure coding.
 //       Since any sub-matrix of a Cauchy matrix is invertable.
 //
@@ -53,14 +52,14 @@
 //
 // Creating and using an encoder
 //  var bytes []byte
-//  params := erasure.ValidateParams(10, 5, erasure.Cauchy)
+//  params := erasure.ValidateParams(10, 5)
 //  encoder := erasure.NewErasure(params)
 //  encodedData, length := encoder.Encode(bytes)
 //
 // Creating and using a decoder
 //  var encodedData [][]byte
 //  var length int
-//  params := erasure.ValidateParams(10, 5, erasure.Cauchy)
+//  params := erasure.ValidateParams(10, 5)
 //  encoder := erasure.NewErasure(params)
 //  originalData, err := encoder.Decode(encodedData, length)
 //