add help print to mk_prepare_receptor

[diogomart]

Adds help message for users who are used to -o flag in mk_prepare_receptor in older versions. Before v0.6.0, -o sets the PDBQT output filename, but in v0.6.0, it sets a default output basename that --write options rely on.

If someone runs -o output.pdbqt -j, the -j flag triggers writing a file named "output.pdbqt.json", but no PDBQT files are written. The added help message, shown below, is printed if the output basename ends with ".pdbqt" and --write_pdbqt was not used.

PDBQT files were NOT written. Use -p/--write_pdbqt for that.
Note that -o/--output_basename just sets a default for --write flags

[rwxayheee]

This looks good, thank you! I wanted to drop the print(f"{templates=}") line too, thanks for doing that. I approve the PR