Update a step of the SFA algorithm.

Calculates the first derivative of signal data

1 | ```
sfaTimediff(DATA, axType = "ORD1")
``` |

`DATA` |
The matrix of signals for which the derivative is calculated (one column per signal) |

`axType` |
Type of interpolation: "ORD1" (default) first order, "SCD" second ,"TRD" third, "ORD3a" cubic polynom |

matrix `DATA`

- `DATA`

contains the derivative signals, with the same structure as the input data.

setting axType to invalid values will lead to first order interpolation.

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