X, S, Y, STY definitions

[dalito]

This issue makes the definition of selectivity universal. It was linked to photo-catalysis before. Moreover, it adds

• converison
• yield
• space-time yield
• catalyst performance measure
• reactor performance measure
• chemical conversion performance measure

Closes #46

[dalito]

Thanks for spotting the typos! It turned out that spell checking was off for turtle-files in my editor.

[nmoust]

@dalito: Thank you for the contribution. I will accept the changes.