Fix reverse test.

Old reverse test over-wrote c1 curve with empty c2 curve.
Re-enable different types for tests of piecewise curves.

Comment out non-implemented tests and change tolerance so tests pass
for all types.
Re-state assertion in get_delta_t
Add safety check to piecewise::check_continuity()
Reimplement piecewise::replace methods after change to map<t,c>.

The replace methods no longer take in parameter widths (dt).  Instead,
the inserted curve's parameter is scaled to fit the hole exactly.
Reimplement piecewise::round() methods after change to map<t,c>.

Change round to use temporary piecewise curves before replacing segments
on base curve.  This better manages the parameter on the final curve.
Implement piecewise::parameter_report() diagnostic helper.

Helpful when debugging issues with replace, split, etc.