Add missing header file

[ashao]

When including scalarfield.h into an application, MetadataBuffer was never defined. Add an explicit include to the header file to ensure that applications can build.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.62%. Comparing base (4937fbd) to head (6938651).
⚠️ Report is 21 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff              @@
##           develop     #530       +/-   ##
============================================
- Coverage    95.92%   57.62%   -38.31%     
============================================
  Files           10      147      +137     
  Lines          884    11908    +11024     
  Branches         0     1137     +1137     
============================================
+ Hits           848     6862     +6014     
- Misses          36     4895     +4859     
- Partials         0      151      +151     

Files with missing lines	Coverage Δ
include/scalarfield.h	100.00% <ø> (ø)

... and 136 files with indirect coverage changes

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[MattToast]

LGTM!! Thanks for the catch!