clean up environment

[insilentio]

in my opinion, it doesn't make sense to create all these variables as they are not needed later on and just clutter the environment. I would just pipe everything, and sometimes (when not used later on) pipe also directly into the plot - makes it very clear to the reader what and what for it is done. But I guess this is a pretty different pov and it is anyway throughout the code; if at all we would need to change it everywhere.

_Originally posted by @insilentio in https://github.com/StatistikStadtZuerich/BevSzen/pull/21#discussion_r864648408_

parameters with p$ piping where possible rm(ls...)

[insilentio]

additionally, prepare for package release: use functions throughout instead of sourcing files

[insilentio]

done