added len method on state store

[cel055]

No description provided.

[codecov]

Codecov Report

Merging #256 (d23ef71) into master (14b31ae) will decrease coverage by 0.06%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #256      +/-   ##
==========================================
- Coverage   78.58%   78.52%   -0.06%     
==========================================
  Files          22       22              
  Lines        1910     1914       +4     
==========================================
+ Hits         1501     1503       +2     
- Misses        409      411       +2     

Files	Coverage Δ
cognite/extractorutils/__init__.py	100.00% <100.00%> (ø)
cognite/extractorutils/statestore.py	90.65% <50.00%> (-0.46%)	⬇️

... and 2 files with indirect coverage changes