[BioPython] blast output xml

[Peter]

In Biopython 1.42, when calling standalone blast we defaulted requesting 
plain text output.

In Biopython 1.43, when calling standalone blast we default requesting 
to XML text output.

Around this time, the NCBI made a big change to how they produced XML 
output for multiple queries, and we had to change the way the NCBIXML 
library was used in Biopython 1.43 - and updated the documentation to match.

Michiel De Hoon wrote:
 >
 > The description in the cookbook is correct for Biopython 1.43.
 >

Maybe we should have added an explicit "you need Biopython 1.43 or 
later" rider on the Blast chapter?

> As a note to the developers:
> We should ask ourselves if distributing packages for various linuxes is
> really a good idea. Installing Biopython isn't all that hard, and the
> packages for linux tend to get behind as new Biopython versions come out,
> causing confusion for our users.

I think having the distributions provide (current) Biopython is great, 
as it is much simpler for new users to install it and its dependencies 
this way.  However, because of the problem of out of data packages, we 
should try and get in touch with the main distribution maintainers and 
try and bring them up to date more quickly.

Peter