plpython: most important parts rewritten to create plpython function
pgmapcss.eval: new infrastructure for python eval functions
Re-implemented eval-functions in plpython

- add 'import re' to check function
- add 'object' to "current" data structure
compile eval(): output compiled code for python
pgmapcss.eval: function for automated tests
compile function check(): re-activate stuff from form plpgsql mode

- @default_other, @values, @postprocess, @depend_property
- order results by object-z-index
Eval functions: improve structure ; add parameter --eval-tests
Fix some eval-functions + tests
Compiler (condition, value): make sure key exists before accessing

- or use dict.get()
eval zoom: bugfix
Merge branch 'master' into plpython
Merge remote-tracking branch 'origin/plpython' into plpython
eval line merge: re-implement in plpython + tests
Compile Condition(s): drop parameter prefix
Compile Properties: bugfix
Eval Test Function: add to_float() function
re-implement more eval functions

- geom functions: buffer, line, line_interpolate_point, line_length,
  line_locate_point, line_part
- color functions: deluma, lighten
- more tests for metric
Eval Test Function: make sure return value is a string
eval functions: return float values with highest precision
re-implement more eval functions

- natsort, random, switch, sqrt
- geom: rotate, translate