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/donut/encoder.go

@@ -23,41 +23,21 @@ import (
 
 // encoder internal struct
 type encoder struct {
-	encoder   *encoding.Erasure
-	k, m      uint8
-	technique encoding.Technique
-}
-
-// getErasureTechnique - convert technique string into Technique type
-func getErasureTechnique(technique string) (encoding.Technique, *probe.Error) {
-	switch true {
-	case technique == "Cauchy":
-		return encoding.Cauchy, nil
-	case technique == "Vandermonde":
-		return encoding.Cauchy, nil
-	default:
-		return encoding.None, probe.NewError(InvalidErasureTechnique{Technique: technique})
-	}
+	encoder *encoding.Erasure
+	k, m    uint8
 }
 
 // newEncoder - instantiate a new encoder
-func newEncoder(k, m uint8, technique string) (encoder, *probe.Error) {
+func newEncoder(k, m uint8) (encoder, *probe.Error) {
 	e := encoder{}
-	t, err := getErasureTechnique(technique)
+	params, err := encoding.ValidateParams(k, m)
 	if err != nil {
-		return encoder{}, err.Trace()
-	}
-	{
-		params, err := encoding.ValidateParams(k, m, t)
-		if err != nil {
-			return encoder{}, probe.NewError(err)
-		}
-		e.encoder = encoding.NewErasure(params)
-		e.k = k
-		e.m = m
-		e.technique = t
-		return e, nil
+		return encoder{}, probe.NewError(err)
 	}
+	e.encoder = encoding.NewErasure(params)
+	e.k = k
+	e.m = m
+	return e, nil
 }
 
 // TODO - think again if this is needed