add filter for accelerators and pytests

[trz42]

This is a first PR towards better support for building for GPUs. It includes the following changes/additions:

• a new filter component accelerator (that can be added to to bot: build ... commands)
• adjusted processing of filter component names so that components must be given with at least three characters
• improved definition and use of strings for raising exceptions if anything went wrong with providing filters
• fixed code for removal of filters (the code was neither used nor tested, thus we were not aware that it would not always work)
• added tests for the removal of filters and for the new filter component (accelerators)
• updated tests that were broken by code changes

[laraPPr]

@trz42 I have had a look and seems to look good. The only thing that is add in this pr is that you can pass accelorator in the bot command nothing more right? All the other stuff will than in an other PR right?

[laraPPr]

lgtm