add MultiArrayN for small N

[cseed]

Use in MendelErrors for trio genotypes in place of existing Array[Array[GenotypeType]].

Sketch:

import scala.reflect.ClassTag

class MultiArray2[T](val n1: Int,
  val n2: Int,
  a: Array[T]) {
  require(a.length == n1*n2);

  def apply(i: Int, j: Int): T = a(i*n2 + j)

  def iterator: Iterator[T] = a.iterator

  // FIXME def reshape(...): MultiArrayM[T]
}

object MultiArray2 {
  def fill[T](n1: Int, n2: Int)(elem: => T)(implicit tct: ClassTag[T]): MultiArray2[T] =
    new MultiArray2[T](n1, n2, Array.fill[T](n1 * n2)(elem))
}

[cseed]

Done for N=2: f65627e