needed a function to 'clear' an instance of an Aligner

[dieuska]

Instead of making a new instance, should it be possible to clear all information of an Aligner-instance, so it can be reused in code.

f.e. Creating an instance of the Aligner on initilaisation of the processe and loading/removing data (thematic/reference) to the same instance.
Or always creating a new instance?

[dieuska]

probably the reason was found why reusing the Aligner and loading new thematic_data was not working. When loading new thematic_data the thematic union was not reset to None, and the former thematic_union was used. This is fixed as a bug

[dieuska]

To check if this is fixed

[dieuska]

checked that this is fixed with #122 commit