Change #include of FlexibleInput header and source file

[maxschalz]

Change flexible_input.cc to flexible_input.tpp to clearly mark it as a template implementation. Then place #include "flexible_input.tpp" at the end of the header file and import said header file in, e.g., miso_enrich.h etc.

See https://stackoverflow.com/a/495056/15268739

[maxschalz]

This will probably not work because the compiler will not find the correcft files. Another potential idea is to add the line template class FlexibleInput<double>; to the end of the source file, see here.