maint: Clean up m-files to follow Octave coding conventions. Try to trim long lines to < 80 chars. Use '##' for single line comments. Use '(...)' around tests for if/elseif/switch/while. Abut cell indexing operator '{' next to variable. Abut array indexing operator '(' next to variable. Use space between negation operator '!' and following expression. Use two newlines between endfunction and start of %!test or %!demo code. Remove unnecessary parens grouping between short-circuit operators. Remove stray extra spaces (typos) between variables and assignment operators. Remove stray extra spaces from ends of lines.

--- a/scripts/signal/fftfilt.m
+++ b/scripts/signal/fftfilt.m
@@ -185,7 +185,7 @@
 %! y  = fftfilt (b, x);
 %! assert (y0, y, 55*eps);
 
-%% Test input validation
+## Test input validation
 %!error fftfilt (1)
 %!error fftfilt (1, 2, 3, 4)
 %!error fftfilt (ones (2), 1)