better embedding & normalization

Functions for improved image embedding and normalization for source
Gaussian convolution.  Instead of embedding the source image in zero,
new functions embed in the average value of the image perimeter or in a
periodic continuation of the image.
Added function to translate atomic symbols to atomic numbers.

Function to translate from atomic symbol "C", "Sn", etc. to atomic
number.  Useful for translating CIF or VASP POSCAR files into Kirkland
XYZ.