Split validation from the construction of custom qadapter

To solve issue vre-language#247 (closed) we need to call get_custom_qadapter() without validation that requires that the numerical resources are numeric types (in default SLURM units). The best way is to write a function that does only the validation and this function to be called optionally by get_custom_qadapter() on start. Also this new validation function can be called independently.