Merge branch 'no_specific_counters' into 'master'

No specific counters and leaner cell structure.

Implements #225 and #227.

While re-packing of the cell structure I have made the following changes:

- All the depth-related things are now chars. Depth beyond 256 is ridiculous.
- All the binary things (e.g. c->split) are now chars.
- The number of tasks associated with a cell is a short int.
- The hold locks are now chars. @nnrw56 that's the only part I worry about. Any objection to this ?

In combination with the removal of the statistics, the cell structure has gone from 576 bytes to 416 bytes. Might help with the memory bandwidth. 

All fields are now individually documented using Doxygen.

See merge request !270