optim_doc: gjp

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### 5.2 Gauss-Jordan pivoting

#### Helptext:

    m = gjp (m, k[, l])

    m: matrix; k, l: row- and column-index of pivot, l defaults to k.

    Gauss-Jordon pivot as defined in Bard, Y.: Nonlinear Parameter
    Estimation, p. 296, Academic Press, New York and London 1974. In
    the pivot column, this seems not quite the same as the usual
    Gauss-Jordan(-Clasen) pivot. Bard gives Beaton, A. E., 'The use of
    special matrix operators in statistical calculus' Research Bulletin
    RB-64-51 (1964), Educational Testing Service, Princeton, New Jersey
    as a reference, but this article is not easily accessible. Another
    reference, whose definition of gjp differs from Bards by some
    signs, is Clarke, R. B., 'Algorithm AS 178: The Gauss-Jordan sweep
    operator with detection of collinearity', Journal of the Royal
    Statistical Society, Series C (Applied Statistics) (1982), 31(2),
    166--168.