Spherical coordinates: inconsistent notation

Make sure:

• use
  \theta
  instead of
  \vartheta
• use the order
  (r, \varphi, \theta)
  everywhere.

assigned to @mmtop

mentioned in commit ff6d41f8

mentioned in merge request !19 (merged)

mentioned in commit 944235a6

mentioned in commit b838ec54

mentioned in commit 2ece56cb

closed with commit b838ec54

closed with merge request !19 (merged)