Perf optimisation for CovSeIso

[danielkorzekwa]

This test takes 5 sec to run, Different impl of CovSeIso from bayes-scala takes 0.7 sec to execute.

 @Test def perf_test_1d_df_dSf = {

    val x1 = Array(10d)
    val x2 = Array(2d)
    (1L to 2000L * 50 * 50).foreach(_ => covFunc.df_dSf(x1, x2))

  }