May need a slight change to better handle options for meshes of order >2

moved all string options to std::string

*****************************************************************
WARNING: this has the potential to break things in subtle ways...
         PLEASE TEST AND REPORT ANY WEIRDNESSES
*****************************************************************

- fix bb computation to avoid model jumping around when adding points in z=0 plane
- pass interpol matrices as ref

Third phase of code refactoring: move callbacks where they belong
& drastically reduce namespace pollution.

There's no new functionality for now: PLEASE TEST THAT EVERYTHING
WORKS AS IT USED TO BEFORE.

There's no new functionality for now: PLEASE TEST THAT EVERYTHING
WORKS AS IT USED TO BEFORE.