PV_AVA and PV_NUM and stress tensor for FIST

[Teodoro Laino]

Dear all,

a small modification to cp2k was needed in order to make uniform the  
behavior of
FIST compared to the one of the other methods available.
Now every time you need the stress tensor (NPT or similar ensembles)  
you've to specify
the keyword STRESS_TENSOR in the &MD section.
STRESS_TENSOR can have the following arguments:
NONE, ANALYTICAL,  NUMERICAL, DIAGONAL_ANALYTICAL, DIAGONAL_NUMERICAL

The old keywords: PV_AVA and PV_NUM and PV_DIAG have been deleted.
If you forget to switch on the STRESS_TENSOR asking for an NP*  
ensemble the program
will stop with a message.

Specifically for FIST this will make possible for all runs in which  
the stress tensor is not needed to save some cpu time (next commit
will specifically affect this but will be totally transparent to  
users..)

Again, I'm sorry if this causes you any problems but the hope is that  
these kind of changes will
be fewer and fewer until all the old stuff will be updated.

As usual, let me know in case of problems.
Cheers
teo