extremely experimental RRD step stretching
Further chop up rrd_modify_r in order to integrate step changes into DS/RRA
modifications.
improve step resizing, this still is experimental
Take care to change CDPs when changing step size and change resize semantics
- all RRA modifications are now done AFTER resizing
Make modify command available using RRDs
improved rrdmodify documentation
Revert "Make modify command available using RRDs"

This reverts commit c77b6b7a567bd787baa98a9e459bd64e9ef39154.
This cs should not have been on this branch, but because the branch was
pushed before, it can only be reverted...
Merge pull request #452 from stamfest/rrdmodify-master

Rrdmodify master