Expose domain() for pchip

[mborland]

We already have this function for the other interpolators of the hermite family. This was the only one missing the domain() function.

Closes: #1380

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.35%. Comparing base (9c00175) to head (9f8cf5c).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1381   +/-   ##
========================================
  Coverage    95.34%   95.35%           
========================================
  Files          825      825           
  Lines        68196    68203    +7     
========================================
+ Hits         65022    65034   +12     
+ Misses        3174     3169    -5     

Files with missing lines	Coverage Δ
include/boost/math/interpolators/pchip.hpp	89.79% <100.00%> (+0.43%)	⬆️
test/pchip_test.cpp	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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