Compute process standard deviation

This is an internal function to compute the process variance.

1 | ```
process.var.mle(sigma2, s, beta.bars, N, n)
``` |

`sigma2` |
Estimated imprecisions for each method in an N vector. |

`s` |
Variance-covariance N x N matrix. |

`beta.bars` |
Estimates or hypothesized values for the N betas. |

`N` |
No. of methods. |

`n` |
No. of items. |

See Jaech p. 186 equations 6.37 - 6.3.10.

Estimated process variance.

Richard A. Bilonick

Jaech, J. L. (1985) *Statistical Analysis of Measurement Errors*. New York: Wiley.

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