slice/append behaves stupid for inferred curves

example:

```python
from keyframed import SinusoidalCurve
c0 = SinusoidalCurve(wavelength=20)
c0_halfwave = c0[:10]
#c0.plot(20)
#c0_halfwave.plot(20) # hmm...

c1 = SinusoidalCurve(wavelength=40)
c1_halfwave = c1[20:]

c2 = c0_halfwave.append(c1_halfwave)
c2.plot(20) # lol. yeah, that's no good
```

I think what I need to make this work is to change slicing to use a wrapper that maps the new `t` to the old `t`, rather than changing `t` internally. This suggests to me that the `append` operation probably also needs to change. the idea behind append originally was to construct a curve object. actually... this could potentially still work, i'd just need to push the mapping operation onto the keyframe object. this would also make it a lot easier to compose mappings