Return the first principal component of residuals of `S`

given
the components `P`

.

1 | ```
firstpcres(S, P)
``` |

`S` |
correlation/covariance matrix |

`P` |
component matrix |

numeric vector; actually, the first eigenvector of
*S - A' (A P)^(-1) A* where *A := P' S*.

`sca`

, also for references

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