doc: Use more coding conventions in Object Oriented reference example code. * examples/code/@FIRfilter/FIRfilter.m, examples/code/@FIRfilter/FIRfilter_aggregation.m, examples/code/@FIRfilter/subsasgn.m, examples/code/@FIRfilter/subsref.m, examples/code/@polynomial/display.m, examples/code/@polynomial/double.m, examples/code/@polynomial/get.m, examples/code/@polynomial/mtimes.m, examples/code/@polynomial/numel.m, examples/code/@polynomial/polynomial.m, examples/code/@polynomial/polynomial_superiorto.m, examples/code/@polynomial/polyval.m, examples/code/@polynomial/roots.m, examples/code/@polynomial/set.m, examples/code/@polynomial/subsasgn.m, examples/code/@polynomial/subsref.m: Remove "Function File" from deftypefn. Use names of variables in error messages. Surround invalid items in error messages with single quotes. Use meaningful variable names.

function n = numel (obj, idx)
  n = 1;  # always produce an array.
endfunction