Profiling: consider cross-database merge for KMCP

[jfy133]

Description of feature

Merge search results from multiple databases

Input:
  *. Searching results of the same reads in different databases.
  *. The order of multiple input reads files should be the same during searching.
  *. When only one input given, we just copy and write to the input file.
     This is friendly to workflows which assume multiple inputs are given.

We don't currently support this for other tools so we should think if we should include it, but we could consider it as an optional post-processing step, as we explicitly design taxprofiler to allow running of multiple databases