Support sparse inputs in GTIL

[codecov[bot]]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (07f9b1e) 84.20% compared to head (c66648a) 84.35%.

Files	Patch %	Lines
python/treelite/gtil/gtil.py	81.81%	4 Missing :warning:
src/c_api/gtil.cc	76.92%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## mainline #551 +/- ## ============================================ + Coverage 84.20% 84.35% +0.15% ============================================ Files 71 71 Lines 6071 6130 +59 Branches 510 514 +4 ============================================ + Hits 5112 5171 +59 Misses 959 959 ```

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