Add tests and set correct return type for `time`

[danielhuppmann]

Description of PR

This PR adds several tests for the IamSlice feature. To ensure consistency, the tests are implemented directly as alternatives to the filter() tests.

It also implements the different return-type for the time attribute (pd.Index, all other dimension-attributes are lists).

[codecov[bot]]

Codecov Report

Merging #642 (59b5161) into add-iamslice (8fe4376) will decrease coverage by 0.6%. The diff coverage is 94.7%.

@@              Coverage Diff               @@
##           add-iamslice    #642     +/-   ##
==============================================
- Coverage          94.4%   93.7%   -0.7%     
==============================================
  Files                57      57             
  Lines              5698    5701      +3     
==============================================
- Hits               5380    5344     -36     
- Misses              318     357     +39     

Impacted Files	Coverage Δ
pyam/core.py	93.7% <75.0%> (+0.2%)	:arrow_up:
tests/conftest.py	100.0% <100.0%> (ø)
tests/test_filter.py	100.0% <100.0%> (ø)
tests/test_time.py	100.0% <100.0%> (ø)
tests/test_unfccc.py	40.0% <100.0%> (-60.0%)	:arrow_down:
pyam/unfccc.py	24.4% <0.0%> (-65.4%)	:arrow_down:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 8fe4376...59b5161. Read the comment docs.

[danielhuppmann]

Implemented the suggested change. I noticed that this changed the "name"-attribute of the returned pd.Index object (from None to time). This actually seems to be preferable behavior, so I also implemented the minimal changes to make IamDataFrame.time behave in the same manner.