Tweaked the xml blocks usage information, added where missing. Added workarounds for color type step and smoothstep, since i'm told only PRman supports both float and color types, but available RI specs (3.2.1) mention only float type, so in other renderers it'll operate on a component by component basis.
Removed E and PI number blocks, and replaced them instead by a "Useful constants" block, with the values of E, PI, PI/2, 2PI, 1/PI, as defined in shrimp_helpers.h's symbols.