Refactor for a more modern API

[mattcieslak]

this library has been using raw pointers throughout. It would be better to use smart pointers. This PR also includes some other c++ modernizations now that we're building for c++17

[codecov]

Codecov Report

❌ Patch coverage is 83.50109% with 377 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.03%. Comparing base (ea9ebcb) to head (9bea625).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
include/trx/trx.tpp	80.88%	151 Missing ⚠️
src/nifti_io.cpp	30.60%	127 Missing ⚠️
src/trx.cpp	79.92%	53 Missing ⚠️
tests/test_trx_anytrxfile.cpp	92.16%	34 Missing ⚠️
include/trx/trx.h	85.00%	6 Missing ⚠️
tests/test_trx_io.cpp	96.52%	5 Missing ⚠️
src/detail/dtype_helpers.cpp	98.61%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #27      +/-   ##
==========================================
- Coverage   85.67%   84.03%   -1.64%     
==========================================
  Files           9       13       +4     
  Lines        2917     4724    +1807     
  Branches      460      729     +269     
==========================================
+ Hits         2499     3970    +1471     
- Misses        418      754     +336     

Flag	Coverage Δ
linux	84.10% <83.39%> (-1.83%)	⬇️
macos	83.92% <83.40%> (-1.58%)	⬇️

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