emit deprecation warnings for custom logging functions

[pmeier]

This foreshadows #434

[codecov]

Codecov Report

Merging #442 into master will decrease coverage by 0.0%.
The diff coverage is 88.2%.

@@           Coverage Diff            @@
##           master    #442     +/-   ##
========================================
- Coverage    90.7%   90.7%   -0.1%     
========================================
  Files          63      63             
  Lines        3091    3106     +15     
========================================
+ Hits         2806    2819     +13     
- Misses        285     287      +2     

Impacted Files	Coverage Δ
pystiche/optim/optim.py	96.3% <80.0%> (-1.1%)	⬇️
pystiche/misc.py	95.9% <100.0%> (+0.1%)	⬆️

---

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 dd34c10...7f32b8a. Read the comment docs.

[pmeier]

Lint error is unrelated and was fixed in #443.