LATEX : How to define and use a common nomenclature across the documents

[KikeM]

References:

• https://www.amherst.edu/system/files/latextips.pdf
• https://www.math.tamu.edu/~harold.boas/courses/math696/math-macros.html

[KikeM]

\newcommand{\christoffel}[3]{\ensuremath{\Gamma^{#1#2}_{#3}}}

[KikeM]

I am using few commands for the moment, so not big issue if I need to copy and move them around.

Related to #15.