add new implementation for "reduce_dimension" process

[MichaelBrueggemann]

What

This new implementation of reduce_dimension provides some new Features:

• are cleaner layout of the code
• support for adding "custom reducer functions". This approache uses "eval()/parse()" in combination with the "Blacklist"-approach suggested in Safety Issue with "run_udf"!!! #81. This allows users to give their own reducer functions beyond the scope of pre-defined reducers.
  • This also is a work-around for gdalcubes current implementation of reduce_time() as it's not possible to rename the bands of the new cube created by this process if you use a pre-defined reducer
• now only "time" is supported as an argument for declaring a temporal reduction (instead of also supporting "t") to make the function more understandable
• it adds the option to keep bands that are included in the datacube before calling this process

Suggestion

I also suggest to move the implemtion of this function for the argument "bands" into it's own function, as it's confusing on whats the purpose of the function reduce_dimension.