Epi: added Eric's files unchanged (#567).
Epi: reformat code for readability.
Epi: rename functions, export them, add "sir" class.
Epi: fixed renaming bug of no_sim.
Epi: added a test case.
Epi: added a benchmark.
Epi: add random seed to benchmark.
Epi: speed up SIR model by clever updating.
R: added a test case for simple psumtree draws.
Epi: C implementation of SIR, not final yet.

The result vectors will be restructured, to match the initial
structure.
Epi: sir rewritten to use the old style of data structure in the result.
Epi: brush up R plotting function.
Epi: fixed a typo in plotting.
Epi: ooops, added missing sir.c file.
Epi: added C docs to SIR model.
Epi: deallocate memory on error, properly.
Epi: ooops, add missing header file.
Epi: added R docs.