implement Area and Volume methods for Section Objects

[hanagali]

Overview of the feature

Segment objects have methods called .area() and .volume() which return the area and volume of that segment when called.
The Section objects, however, do not have these methods, and to get the values they must be calculated manually by summation of the Segment returns.
This would enable automatic calculation of Section.area() and Section.volume().

The corresponding documentation should also be updated accordingly, as it currently does not clearly state or mention the above available methods for Segment.

Foreseeable Impact

Easier access to relevant data and properties of Section objects, fewer manual calculations by summing Segment data (and thus, less chance for human error)

• Area(s) of change: NEURON, CoreNeuron, online documentation ( https://nrn.readthedocs.io/en/latest/index.html )
• Possible issues: level of accuracy of return values, computation time

[clarenznet]

I could do the area and volume calcs