Traces as numpy structured arrays

[lbluque]

Is your feature request related to a problem? Please describe.
Please describe the problem you are trying to solve.

In #154 Trace classes where introduced to generalize sampling values during MC. These classes only allow np.arrays as attributes. It seems this is a good opportunity to use numpy structured arrays instead, since they essentially provide all the functionality needed and will also benefit from the better memory layout,

Describe the solution you'd like
Please describe the desired behavior.

Use numpy structured arrays instead of Trace/StepTrace classes