Standardise DSL and UDF interface

Currently, one writes a DSL kernel as

```
@dsl_kernel
def myfun(x, y, z):
return x + y * z
```
and a udf as
```
def myfun(inputs, output, offset):
x, y, z = inputs
output[:] = x + y * z
```

but then uses the same interface to wrap them as ``lazyudf``:

``blosc2.lazyudf(myfun, (x,y,z), dtype=np.float32)``

This seems strange and might be confusing for users.