...by actually making use of the symmetry over the equator. The loop
was incorrectly running over all rings rather than only the northern
half.

On an equidistant cylindrical projection (ECP) grid, we know several
facts about symmetry and locations of pixels, so improve upon the
reference implementation with a method that adds optimizations
appropriate for isolatitude rings.

Completes the symmetry of the existing `synthesize_ecp`.

… functions

Part 1 just adds very basic analytical checks of the reference
implementation.

With increasing infrastructure in the general form (and passing chains
of tests building up to generic implementation), move the slow/specific
reference functions to the tests file to reduce code compiled into the
module itself.

These are useful in implementing linear algebra-like operations on
spherical harmonic coefficients without unnecessarily including nearly
half of the always-zero matrix elements above the main diagonal

This is required to solve for quadrature weights in the spherical
harmonic analysis transform of arbitrary pixelizations.

Handle null vector and exact initial conditions in C.G. better

- Avoid any iteration when initial condition is already sufficiently
  accurate
- Less-than-or-equal tolerance check handles cases when result is
  exact, and therefore avoids a division by zero in constructing the
  iterative update