cLoadedImage: fix InterpoleOfEtape call in cEtapeMecComp.cpp with gcc-10

[trabucayre]

With gcc-10 build fails with

cEtapeMecComp.cpp:(.text+0xdcd): undefined reference to `cInterpolateurIm2D* InterpoleOfEtape<float, double>(cEtapeMEC const&, float*, double*)'

The version of InterpoleOfEtape with typeEL=float, and tBase = double is not available out of the cpp file.
Adding explicitly this signature force the compiler to generate required method.

Fix issue #166