Enable filter for mixed year/time domain

[danielhuppmann]

Please confirm that this PR has done the following:

• [x] Tests Added

Description of PR

Note: this PR is targeted to the feature/year-and-datetime branch that collects all developments related to #596.

This PR extends the filter() method to work with mixed time domain.

[codecov[bot]]

Codecov Report

Merging #612 (138a263) into feature/year-and-datetime (67de2a0) will decrease coverage by 0.0%. The diff coverage is 98.8%.

@@                     Coverage Diff                     @@
##           feature/year-and-datetime    #612     +/-   ##
===========================================================
- Coverage                       94.3%   94.3%   -0.1%     
===========================================================
  Files                             54      54             
  Lines                           5496    5478     -18     
===========================================================
- Hits                            5188    5168     -20     
- Misses                           308     310      +2     

Impacted Files	Coverage Δ
pyam/utils.py	90.9% <85.7%> (-0.7%)	:arrow_down:
pyam/core.py	94.8% <100.0%> (+<0.1%)	:arrow_up:
pyam/time.py	96.0% <100.0%> (+<0.1%)	:arrow_up:
tests/conftest.py	100.0% <100.0%> (ø)
tests/test_core.py	100.0% <100.0%> (ø)

---

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 67de2a0...138a263. Read the comment docs.