Client: add ability to count the number of matches for a filter using binary search over pagination

[ml-evs]

Closes #1924.

This PR implements a workaround for the client to count the number of results returned by a filter using binary search over pagination. When a database does not return meta->data_returned, for whatever reason, the client will now execute the query with a series of probe page_offset values to find the number of matching results, for example, starting with a query for page 1,000,000, and finding no results, the client will then try 1,000. If a result is found on that page, the window has been narrowed to 1,000 to 1,000,000. The window will be reduced logarithmically until each end of the window has the same approximate power of 10, at which point the average value will be taken, e.g., if the query returned 1,001 entries, the trial values would be:

• 10^3 -> 10^6 => int(10^4.5)
• 10^3 -> 10^4.5 => int(10^3.75)
• 10^3 -> 10^3.75 => int(10^3.375)
• 10^3 (1000) -> 10^3.375 (2371) => 3371 // 2 = 1685
• 1000 -> 1685 => 1342
• 1000 -> 1342 => 1171
• etc until reaching 1001.

I think this scheme makes more sense than vanilla binary search or exponential search as OPTIMADE queries of this kind are probably either smallish or largeish, without much middle ground...

Caveats:

• the server must currently implement page_offset; page_number is possible but not yet implemented
• the code for one API must currently be run asynchronously

PR also adds a verbosity flag -vvv that enables some debug printing on the client.

[codecov]

Codecov Report

Attention: 16 lines in your changes are missing coverage. Please review.

Comparison is base (ed27075) 90.81% compared to head (5805ef8) 90.77%.

Files	Patch %	Lines
optimade/client/client.py	79.22%	16 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1925      +/-   ##
==========================================
- Coverage   90.81%   90.77%   -0.04%     
==========================================
  Files          75       75              
  Lines        4661     4728      +67     
==========================================
+ Hits         4233     4292      +59     
- Misses        428      436       +8     

Flag	Coverage Δ
project	90.77% <79.74%> (-0.04%)	⬇️
validator	90.69% <79.74%> (+<0.01%)	⬆️

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