Change the build_xxx_splines methods to accept a const reference parameter

to the data vector rather than a vector parameter. Vectors are, as all
types are, passed by value in C++. Using a vector directly as a parameter
causes the data to be copied. Using a reference to the vector neatly
avoids that problem.

Also tucked fmt/format.h into its rightful place in the include list
since Axel says there's no absolute rule about it being an exception.