Add tmqm dataset

[sgugler]

Together with Stefaan I wrote a loader for the tmQM data set.

In line 192 we made a setup --analogous to the materials project dataset code-- to pass a string as a property (usually an identifying code of the molecule in a data base).

I tested this locally with a script and it worked. What is missing is a way to do it with yaml from the CLI.

[stefaanhessmann]

thanks for the contribution. we can add the config file later