Fix compilation of step-70.

[drwells]

I'm not sure how this ever worked. Unfortunately I only noticed the problem when updating the deal.II AUR package, so we need to patch this tutorial to get things compiling with all up-to-date dependencies.

[tjhei]

Do we need a 9.4.2? :-(

[drwells]

Maybe? A better fix would be to ensure that whichever CI configurations compile examples also have opencascade installed.

[bangerth]

Adding a boost header is uncritical, though one may also just want to use standard C++ facilities to do this. There is only one place in the program where we use boost facilities, namely here:

        const auto extension = boost::algorithm::to_lower_copy(
          cad_file_name.substr(cad_file_name.find_last_of('.') + 1));

A simpler version without boost would be to use this call to std::transform: https://stackoverflow.com/questions/313970/how-to-convert-an-instance-of-stdstring-to-lower-case
If you want, I can make the corresponding change.

[tamiko]

I guess it worked because some transitive dependency did include the right boost header.
Looking at the regression tester [1] and for the deal.II-9.4.1 version - we do indeed compile with opencascade.

I guess I would run into the same problem once I update the regression tester to the latest dependency versions again.

[1] https://cdash.dealii.org/index.php?project=deal.II

[tamiko]

@bangerth Would you mind to make a pull request with a std::transform + std::tolower? I would prefer such a version.