Left right iterator

[nluetts]

Perform integration based on an iterator that is initialized with a curve object and two bounds:

for (xi, yi) in (curve, left_bound, right_bound)
    ... # do something with data points
end

The key is that data points at the left and right boundaries will be interpolated linearly if they happen to fall
between the x-values (just like it was previously done in the trapz function).

This PR also fixes tests and deprecation warnings and prepares for a version 1.0.
Tested Julia version is now 1.10.

[nluetts]

still needs fixing the doctests ...