.prior_info not overwritten

[AdamOrmondroyd]

Found myself thinking about .prior_info today and managed to discover two issues.

1. ndiscarded includes the additional points requested by the root process which aren't included if the number of slave processes doesn't nicely divide nprior-1*. This would lead to the incorrect conclusion that there is an unphysical region even if there isn't.
2. .prior_info doesn't get cleared each time there is a new run, the new numbers are appended to the existing file.

*this isn't quite accurate, still getting my head around why 11-1=10 slave processes would have ndiscarded=210.

e.g. with nprior=nlive=200 and resume=False:

$ mpirun -np 3 python quickstart.py
$ cat chains/gaussian.prior_info
nprior =          200
ndiscarded =          201
$ mpirun -np 5 python quickstart.py
$ cat chains/gaussian.prior_info
nprior =          200
ndiscarded =          201
nprior =          200
ndiscarded =          203
$ mpirun --oversubscribe -np 37 python quickstart.py
$ cat chains/gaussian.prior_info
nprior =          200
ndiscarded =          201
nprior =          200
ndiscarded =          203
nprior =          200
ndiscarded =          235
$ mpirun --oversubscribe -np 11 python quickstart.py
nprior =          200
ndiscarded =          201
nprior =          200
ndiscarded =          203
nprior =          200
ndiscarded =          235
nprior =          200
ndiscarded =          210

This last one I can't understand the 210.