Faster queries

[jclymo]

Hi Cameron,

when I was querying ICSD I wanted to speed it up a bit and also to generate a zip file of the downloaded cifs and this escalated into quite a lot of changes.

• separating the logic of making a query to the ICSD and managing the overall query (i.e. the difference between I want to get 3000 cifs and needing to make multiple calls within the cif limit).
• context manager for login / logout
• collect cifs into zip file
• multithreaded calls to ICSD
• managing the multiple logins required for the separate calls
• allowing generic search strings to be passed in rather than dict of search_term: value because one of the queries I wanted to make couldn't be naturally expressed using only 'and' or 'or'. Using a dict of search terms to create the search string is still there but as a separate method now.

I have tried to maintain the existing functionality where possible but the interface has changed a bit. What do you think?

Jude