examples/liquid: specify integration rather than total steps

The total number of integration steps is pretty unhandy and not
physically meaningful since it depends on the integration timestep.

The change has already been introduced in commit b5cfb5c silently (by
accident), and this commit fixes a failing integration test, see e.g.,

http://my.cdash.org/testDetails.php?test=11507126&build=493412
observables/…/wavevector: fix bug with multimap keys

The comparison predicate less_tol() used to sort wavevectors by their
magnitude did not induce a strict weak ordering. The tolerance was
simply chosen too large since the stored keys are taken from the
wavenumber grid provided (and not from the magnitude of the wavevectors
found). The issue was introduced in commit 1d73ef8b.

The issue has been detected by a failing assertion in a test run:
http://my.cdash.org/testDetails.php?test=11507122&build=493412