Write a validphys function to process fktables

Should have a boolean mask to select the flavours, the xgrid and sigma.

sigma should have the  following dimensions for hadronic:

```
ndata x nchannels x nx x nx
```

Where nchannels loops over the active partonic channels, which are encoded in the file.

For DIS, it is:

```
ndata x nflavours x nx 
```
