Add numerical quadrature namespace and trapezoid integrator + unit test

[Reneh107]

I have made a numerical quadrature folder and the corresponding namespace and I added a trapezoid integrator + unit test

I would like an opinion on the following:

• Did I implement the baseclass is in a favorable way?
• Do we want to save data in the integrator class? The trapezoid integrator is now constructed using data and I have two integrate functions, which can be used with and without sending data.
• We can also just use functions instead of classes, but maybe more functions will be implemented that belong to the method. For example, a measure of the error? I prefer classes instead of single functions.

An additional option would be to add a function: integrate between bounds

[DominicDirkx]

Hi Rene,

I've had a first look at the code, and I think we can get this into Tudat pretty quickly. I've made some small changes to the code, see

https://github.com/DominicDirkx/tudat/tree/Reneh107-add_numerical_quadrature

I've kept the class structure, but added a free function for the trapezoid quadrature, allowing the user to choose which one to use.

[DominicDirkx]

I've added a few bits to the unit test to verify the order/symmetry of the method and committed it to teh repository. Thanks for all your hard work Rene!