Add a proper length attribute

[danielhuppmann]

Description of PR

This PR fixes the __len__ attribute, which should only count the timeseries data points actually included in the slice.

[codecov]

Codecov Report

Merging #643 (5326d92) into add-iamslice (8fe4376) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@             Coverage Diff              @@
##           add-iamslice    #643   +/-   ##
============================================
  Coverage          94.4%   94.4%           
============================================
  Files                57      58    +1     
  Lines              5698    5702    +4     
============================================
+ Hits               5380    5384    +4     
  Misses              318     318           

Impacted Files	Coverage Δ
pyam/core.py	93.5% <100.0%> (+<0.1%)	⬆️
tests/test_slice.py	100.0% <100.0%> (ø)

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