BUGFIX: fix 1st order CRFB handling

For when the user tries to synth a 1st order CRFB modulator -
which happens to be the default form, so it seems worthwhile to
handle this gracefully.
FIX: xlim can be a 1d array.
BUGFIX: ensure no zero-length arrays are ever returned.
TEST: remove tests from _circ_smooth.py.
TEST: remove tests from _clans.py