operations for gaussians

N1 + N2

http://www.math.s.chiba-u.ac.jp/~yasuda/statB/seiki_wa_kei2.pdf
http://physmath.main.jp/src/st-normal-distribution-reproductive-property.html

N1 * N2

http://blog.jafma.net/2010/11/09/the-product-of-two-gaussian-pdfs-is-not-a-pdf-but-is-gaussian-a-k-a-loving-algebra/

convolution

*