tests: Add feq() function to util

* tests/util.lua (feq): New function; compares two float numbers using a
given epsilon.
maint: Add nclaux.timetoseconds() function

* dietncl/nclaux.lua (timetoseconds): New function; returns the number
of seconds denoted by a given NCL time string.
* tests/test-nclaux-timetoseconds.lua: Checks nclaux.timetoseconds.
maint: Add filter.prenorm (partial)

* dietncl/filter/prenorm.lua: New file; pre-normalization filter.
* tests/test-filter-prenorm.lua: New file; checks filter.prenorm.
* Makefile: Updated accordingly.