More useful links added

The useful links section of the sidebar now contains links to the GeoReM database and a pdf of K, L, and M X-ray emission lines.

blogroll and bad links have also been culled or updated.

The useful links section is mostly designed as a link hotline for numbers or formulae I can't remember but want to look up in a hurry. It is designed mostly for mineral chemists. If there is anything that ought to be there but isn't, let me know.