added LINEARFIT SPLINELINEARFIT AKIMASPLINEFIT

[pi-r-p]

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[randomboolean]

• The naming has to be revised since Akima and spline interpolators are not linear interpolators.
• I believe it is a better idea to separate the fit (obtaining function parameters) and the creation of the function in two speerate functions, in the same manner it is done in the main library with POLYFIT and POLYFUNC
• This pose the question of the naming convention given to functions of this extension, since for now, BICUBICFIT, TRICUBICFIT and MICROSPHEREFIT outputs directly a function, unlike POLYFIT.
• Also, POLYFIT and POLYFUNC are 1-d interpolators on the ticks, so maybe we would have to name the functions of this PR something like: VALUEPOLYFIT, VALUEPOLYFUNC, VALUESPLINEFIT, VALUESPLINEFUNC, VALUEAKIMAFIT, VALUEAKIMAFUNC

[pi-r-p]

I will update readme and doc once we converge.
For the moment, I did not created the "FIT" function for snapshot 1D interpolator, I clone the input list.

Imho, FIT is a perfect name for regression, not for an interpolator.