Linting setup + code updates

Reported by clang-tidy.

See https://clang.llvm.org/extra/clang-tidy/checks/readability-implicit-bool-conversion.html

See https://clang.llvm.org/extra/clang-tidy/checks/readability-container-size-empty.html

clang-tidy: use auto to avoid writing the same type on both sides of the assignment operator [modernize-use-auto]

See https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-auto.html

clang-tidy: non-static member functions that can be made static [readability-convert-member-functions-to-static]

See https://clang.llvm.org/extra/clang-tidy/checks/readability-convert-member-functions-to-static.html

See https://clang.llvm.org/extra/clang-tidy/checks/readability-const-return-type.html

See https://clang.llvm.org/extra/clang-tidy/checks/readability-isolate-declaration.html

See https://clang.llvm.org/extra/clang-tidy/checks/modernize-pass-by-value.html

Discovered by clang-tidy.

clang-tidy: remove unnecessary calls to std::string::c_str(), redundant void arguments and explicit calls to String upon return

1. https://clang.llvm.org/extra/clang-tidy/checks/readability-redundant-string-cstr.html
2. https://clang.llvm.org/extra/clang-tidy/checks/modernize-redundant-void-arg.html
3. https://clang.llvm.org/extra/clang-tidy/checks/modernize-return-braced-init-list.html

- move constructors and assignment operators should be noexcept
  https://clang.llvm.org/extra/clang-tidy/checks/performance-noexcept-move-constructor.html
- avoid unnecessary copy on range loop variables
  https://clang.llvm.org/extra/clang-tidy/checks/performance-for-range-copy.html
- avoid unnecessary value parameters of types that are expensive to copy
  https://clang.llvm.org/extra/clang-tidy/checks/performance-unnecessary-value-param.html
- avoid unnecessary copy initialization of types that are expensive to copy
  https://clang.llvm.org/extra/clang-tidy/checks/performance-unnecessary-copy-initialization.html

See https://clang.llvm.org/extra/clang-tidy/checks/modernize-deprecated-headers.html

The following checks were used:

- https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-default-member-init.html
- https://clang.llvm.org/extra/clang-tidy/checks/readability-redundant-member-init.html
- https://clang.llvm.org/extra/clang-tidy/checks/readability-redundant-string-init.html
- https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-equals-default.html

clang-tidy: use '= delete' to prohibit calling of a special member function [modernize-use-equals-delete]

See https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-equals-delete.html

clang-tidy: dynamic exception specification 'throw()' is deprecated; consider using 'noexcept' instead [modernize-use-noexcept]

See https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-noexcept.html

See https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-override.html

See https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-nullptr.html

Fixed inconsistency between the signatures of the declaration and definition of the vectorProduct method

Somehow removing 'const' in the declarations caused a weird
lambda-related crash when launching the CUDA kernels in the
vectorProduct methods...

clang-tidy: const-qualification of parameters only has an effect in function definitions [readability-avoid-const-params-in-decls]

See https://clang.llvm.org/extra/clang-tidy/checks/readability-avoid-const-params-in-decls.html

See https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-using.html

Advantages:

- pure C++ for client code, no "<<< ... >>>"
- hence, clang-format can be used for source code formatting
- the HAVE_CUDA guards are not necessary anymore
- type of the kernel function can be added to exception messages
- easier debugging of kernel launch configurations

...and in a few other places to avoid compiler errors/warnings.