Cleanup several things.

* Fix some build rules so that things work in parallel.
* Remove some old code.
* Partly cleanup `attenuation_compute_param.c` as mentioned in #13, though some of the NR code still needs replacing.
* Use MPI_STATUS_IGNORE as in #15.
* Fix a few of the simpler warnings. (Not sure I want to go looking through all of them just yet.)
* Remove Pyre from the build, as it's not used for anything.

In related terms, I'm wondering if I can:
* Remove `DATA/Database00000_left_edge_only`, `DATA/Database00000_right_edge_only`? What are these even used for? I see they're read in the solver, but I'm not sure how they're produced.
* Remove commented lines from `UTILS/visualization/plot_wavefield.pl`? @carltape?