Port from C++11 to C++17

[Lestropie]

Believe the primary relevance was #2297 (QT6)?
But there are other features that we would likely want to change across the code base in the process of transitioning compilation.
Many features can just be made use of whenever a contributor feels like it, but others we might want to adopt across the board.
The goal of this issue is to list & discuss those (so feel free to add to the list).

• Make use of filesystem library
  This may result in reduction or deprecation of core/file/path.h and/or core/file/utils.h
• Nested namespaces
  This I'd personally like to see adopted across the code base in order to remove unnecessary indentation.
• std::byte in place of anywhere that a uint8_t is used as a set of bitwise flags
  Datatype comes to mind, but there are likely others.
  Edit: In addition to bitwise flags, there's also the question of pointers to binary data. On looking around a bit, one suggestion seems to be to use std::byte* for pointers to completely unstructured / untyped binary data, whereas if the implication is that the underlying data will have some type but the handling code is type-agnostic, continue to use void*. But we could equally use std::byte* for any type-agnostic pointers to binary memory. Open to thoughts.
  Edit: For anything for which the size is known at compile time, std::bitset may be better.
• This may also be the right context in which to address Back-end: Remove all char* / char** usages #2111
  Edit: C++17 also has std::string_view; this should perhaps be used in instances of interfacing with some code dependency that uses C-style strings, but I'd advocate removing C-style strings across the board where we have the control to do so.
• Remove custom MR::vector<>, revert to std::vector<>
• Remove custom make_shared<>() and make_unique<>()

[daljit46]

For the first issue about using the filesystem library, what is our intention? For example, do we intend to completely replace the Path namespace with std::filesystem or do we want to essentially make Path make a wrapper around the filesystem library? There are some functions in Path that wouldn't be available in std::filesystem.
Also we should think about whether we want to replace existing instances of std::string with std::filesystem::path.

[Lestropie]

Personally, I would want:

• Any existing usage of std::string referring to a filesystem path to be changed to std::filesystem::path as far upstream as possible, up to and including the type conversion of command-line arguments.
  (Though counter-intuitively there would need to be caution here with respect to image paths due to multi-file numbered image support.
• To remove MR::Path entirely, and remove as much of the MR::File functionality in core/file/utils.h as possible.
  If it's the case that there are certain functions for which there are not standard replacements, we'll need to wait to see what that list looks like and decide where to put it.

[daljit46]

Can we also get rid of our custom MR::make_unique?

[Lestropie]

Yep MR::make_unique<> should disappear along with MR::make_shared<> (will amend OP)

[Lestropie]

@MRtrix3/mrtrix3-devs: Since there will need to be an explicit rebasing process to generate a 3.1.0 containing contents of dev excluding the cmake changes, and this group of changes is incomplete, do we want to keep 3.1.0 at C++11?