Change BUNIT keyword to mention disk center

CoMP L1 `BUNIT` keyword is currently:

```
BUNIT   = '1.E-06 B/Bsun'      / Intensity of Millionths of solar disk brightnes
```

It should mention "millionths of solar disk center intensity". UCoMP's `BUNIT` keyword is going to be:

```
BUNIT   = 'millionths of solar disk center intensity’
```