Initial kw behaviour for cumulative_trapezoid

Hello everyone!

`integrate.cumulative_trapezoid` performs a cumulative integration of fixed samples. The function provides an `initial` kw that can be used to add the starting value for the result. The initial value is just prepended to the result array and not summed to the result. I think this is confusing and should be updated so that the initial value is prepended to the array AND summed to the array.

This was previously discussed in #125, where the concluding thought was: "initial is just a mechanism to keep the output and the input the same length. This has more to do with the "initial" region (i.e. the area under the curve between the non-existent previous value of x and x[0])."

However, cumulative_trapezoid is a cumulative sum of the areas. If we prepend the area from the "initial" region, we would necessarily have to add it to the sum of the subsequent areas. By prepending the value it is now now an initial condition for the integration.

Consider the following practical example:
-   Let y be sampled velocity data of an oscillating object. We can cumulatively integrate to get the displacement of the object. We know the starting position (initial value) is 10m. Without the initial value input, the displacement data will show oscillations around 0m. With the initial value, the displacement should ideally show oscillations around 10m: eg: [10, 8, 5, 7, 10, 12, 13, 11, 5, 10]
-  But, with the cumulative_trapezoid implementation, the first value would be 10m, and the object would then be oscillating around 0m, which is incorrect, eg:
[10, -2, -5, -3, 0, 2, 3, 1, -5, 0]. 
- This behaviour confused me as a user. It can be fixed by prepending and adding the initial value.

Regarding backward compatibility, `initial` would likely be used as "initial=None" or "initial=0". The results for cumulative_trapezoid for these values of `initial` would be identical even if the proposed update is made.

For context, this discussion came up again in #18151 for implementation of `initial` in the proposed `cumulative_simpson` function.

What are your thoughts on this?

Thank you,
Praveer
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