Keep GREL functions preloaded when new functions are loaded in via the `-f` directive

[bjdmeest]

When loading custom FnO descriptions via CLI, the IDLab custom functions remained preloaded, but the GREL functions not. This is not intuitive (see kg-construct/rml-questions#56).
I suggest to keep the grel functions also preloaded. Checking the code, this should be a minor change