Fix duplicates saving with scores for cnn with numpy floats typecaste…

[tanujjain]

Fix to issue #55.

Converts numpy floats to pure python floats to save to json as json can't save numpy floats.

[codecov-io]

Codecov Report

Merging #61 into dev will decrease coverage by 0.07%.
The diff coverage is 93.75%.

@@            Coverage Diff            @@
##              dev     #61      +/-   ##
=========================================
- Coverage   95.17%   95.1%   -0.08%     
=========================================
  Files          16      16              
  Lines         622     633      +11     
=========================================
+ Hits          592     602      +10     
- Misses         30      31       +1

Impacted Files	Coverage Δ
imagededup/methods/hashing.py	99.17% <ø> (ø)	⬆️
imagededup/utils/general_utils.py	100% <100%> (ø)	⬆️
imagededup/methods/cnn.py	97.89% <75%> (-1.03%)	⬇️

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[tanujjain]

@clennan had a look at this with me and finds the implementation ok. So, we're good to merge it.