Reduce compilation time

[mjcarroll]

Noticed that math seemed really slow. Quick look and it seems like we were overusing inline in the Helpers.hh file.

Between moving some of the inlined functions and pruning unneeded headers, we pick up about 30 seconds on my system.

Before:

**** Time summary:
Compilation (165 times):
  Parsing (frontend):           94.3 s
  Codegen & opts (backend):     62.3 s

After:

**** Time summary:
Compilation (165 times):
  Parsing (frontend):           63.9 s
  Codegen & opts (backend):     60.2 s

[codecov]

Codecov Report

Merging #433 (aff2741) into main (3441c1d) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #433      +/-   ##
==========================================
- Coverage   99.69%   99.69%   -0.01%     
==========================================
  Files          73       73              
  Lines        6588     6567      -21     
==========================================
- Hits         6568     6547      -21     
  Misses         20       20              

Impacted Files	Coverage Δ
include/gz/math/graph/Graph.hh	100.00% <ø> (ø)
include/gz/math/Helpers.hh	100.00% <100.00%> (ø)
include/gz/math/detail/InterpolationPoint.hh	100.00% <100.00%> (ø)
src/Helpers.cc	98.63% <100.00%> (+0.42%)	⬆️

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[ahcorde]

  /github/workspace/include/gz/math/Helpers.hh:588:  Line ends in whitespace.  Consider deleting these extra spaces.  [whitespace/end_of_line] [4]