Set name of series returned by aggregate_time()

[danielhuppmann]

Description of PR

Quickfix for #785.

FYI @adrivinca

[codecov]

Codecov Report

Merging #790 (7052f67) into main (fa3fed0) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #790   +/-   ##
=====================================
  Coverage   94.7%   94.7%           
=====================================
  Files         62      62           
  Lines       5965    5966    +1     
=====================================
+ Hits        5650    5651    +1     
  Misses       315     315           

Files	Coverage Δ
pyam/aggregation.py	99.1% <100.0%> (+<0.1%)	⬆️

[adrivinca]

Thanks for the fix. I am not completely clear on what changed in the new version and caused the error. However the remaining questions are:

• why is this issue not tested? should you add test to avoid similar errors happen in the future?
• does this occurs with the other _aggregate* functions? they need to be tested otherwise the package is not stable