Add GetVersion() to Model object

[hcho3]

In downstream applications, it's useful to know which Treelite version produced a particular Treelite model object.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.88 ⚠️

Comparison is base (c47d78e) 85.37% compared to head (06a848b) 84.49%.

❗ Current head 06a848b differs from pull request most recent head 10faef9. Consider uploading reports for the commit 10faef9 to get more accurate results

Additional details and impacted files

@@              Coverage Diff               @@
##             mainline     #485      +/-   ##
==============================================
- Coverage       85.37%   84.49%   -0.88%     
  Complexity         42       42              
==============================================
  Files             112      111       -1     
  Lines            9914     8849    -1065     
  Branches          617       40     -577     
==============================================
- Hits             8464     7477     -987     
+ Misses           1427     1349      -78     
  Partials           23       23              

Impacted Files	Coverage Δ
include/treelite/tree.h	90.66% <ø> (-1.69%)	⬇️

... and 67 files with indirect coverage changes

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