#13 Make common superclass for PwscfBean and ElasPwscfBean

[Daniel Kigelman]

In vlab/commons/pwscf-input-beans, both ElasPwscfBean and PwscfBean have several exact methods.

Right now, they both implement InputBean. While that's nice, it doesn't help with code duplication (can't add method implementations to interfaces).

Ideal solution:

Create new superclass that implements InputBean, and is extended by ElasPwscfBean and PwscfBean.