fix multithreading issue

[thijsjanzen]

Following the discussion here: RcppCore/RcppParallel#245
I added the usage of tbb::task_arena in order to correctly manage the number of threads.

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

❌ Patch coverage is 95.65217% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.86%. Comparing base (fc06d1e) to head (fc9e9c8).
⚠️ Report is 19 commits behind head on master.

Files with missing lines	Patch %	Lines
src/secsse_loglik.cpp	66.66%	1 Missing ⚠️
❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #57   +/-   ##
=======================================
  Coverage   90.85%   90.86%           
=======================================
  Files          19       19           
  Lines        2679     2681    +2     
=======================================
+ Hits         2434     2436    +2     
  Misses        245      245           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.