describe the API

## Trac Ticket #23
**component:** documentation
**owner:** martin
**reporter:** martin
**created:** 2014-02-05 13:43:06
**milestone:** 
**type:** task
**version:** 
**keywords:** 

in bives-core:API

how to extend? see also #2 and #1

and link from
* bives-cellml:wiki
* bives-sbml:wiki
* bives:wiki