fix loo_moment_matching NaN issue

[avehtari]

Fixes #258

• In loo_moment_matching and split_moment_matching replace NaN's in log ratios resulting from -Inf + Inf with -Inf before calling importance_sampling.default()
• in validate_ll() allow -Inf
• update tests

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 92.77%. Comparing base (45c261e) to head (74cb9e3).
Report is 14 commits behind head on master.

❗ Current head 74cb9e3 differs from pull request most recent head 5e4cf37. Consider uploading reports for the commit 5e4cf37 to get more accurate results

Files	Patch %	Lines
R/E_loo.R	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #259      +/-   ##
==========================================
- Coverage   92.78%   92.77%   -0.02%     
==========================================
  Files          31       31              
  Lines        2829     2836       +7     
==========================================
+ Hits         2625     2631       +6     
- Misses        204      205       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[jgabry]

Looks good