move maskers in own folder

[elboyran]

In response to Pranav's suggestion about maskers. I think putting them in utils is not ideal. I think the maskers should get a more prominent place in the package structure. Like:

\ methods
  \ ...
\ maskers
  \ time_series
  \ images
  \ text
\ utils
  \ ...

Originally posted by @cwmeijer in #639 (comment)