Thank you, so that means I need both a platformio.ini and a library.json? That seems quite redundant, but manageable.
The Embedded Template Library compiled fine, but when I try to run pio test I get loads of the following errors:
.piolibdeps/Embedded Template Library_ID930/src/array.h:469:3: error: template with C linkage
template <std::size_t I, typename T, std::size_t MAXN>
^
so I get the feeling I still have something setup wrong.