PDL::Filter::Linear - linear filtering for PDL

```
$a = new PDL::Filter::Linear(
{Weights => $v,
Point => 10});
$b = new PDL::Filter::Gaussian(15,2); # 15 points, 2 std devn.
($pred,$corrslic) = $a->predict($dat);
```

A wrapper for generic linear filters. Just for convenience. This should in the future use DataPresenter.

Also, this class should at some point learn to do FFT whenever it is useful.