Clarify direction of r, theta, phi unit vectors

[liamtoney]

In trying to create a ForceSource object, I was unclear of the direction of the r, theta, and phi hat vectors (mainly the latter two). I found a definition in Nissen-Meyer et al. (2008), Figure 1:

This suggests that — relative to someone standing on Earth's surface — r is positive upwards, theta is positive to the south, and phi is positive to the east. Is this the correct definition? If so, I would kindly suggest including a reference to the paper (as implemented in this PR) for some clarity.

Thanks!

(The RST formatting might be wrong here, I was just guessing.)

[krischer]

r, theta, and phi refer to the standard spherical coordinate system that to my understanding is in use in most of physics (up to the point of it being an official ISO standard). It is the same coordinate system that is also used for moment tensor sources.

I like the idea of adding an explanation. Could you please make two small changes then I'd be happy to merge it:

• Instead of the link to the paper directly spell out what r, theta, and phi are (saves the users a click) and maybe link to the wikipedia page on spherical coordinates systems: https://en.wikipedia.org/wiki/Spherical_coordinate_system
• Could you move it to the top of the file - there is already a box in the top most comment which currently is rendered at the beginning of this page: https://instaseis.net/source.html That way it would be clear that it applies to everything on the page.

[liamtoney]

Cool, I've made those changes.

[krischer]

Thanks a bunch!