trafo.{c,h} pull request

This pull request assumes that ray_tracing.{c,h} is accepted and merged. 

After ray_tracing we can proceed (slowly) towards epi.c - for that we need first to finish:
1. trafo.{c,h}  - the present pull request
2. imgcoord.{c,h}
3. multimed_nlay_

and only then we'll be ready for epi.{c,h}

Meanwhile, this pull request is rather simple: there are four subroutines in trafo.{c,h} with the respective tests in check_trafo.c - tests pass on Mac OS X

Please, review