replace deprecated spreadtab syntax along with proper decimal formatting

[dclift]

Fix for #29:

• replace deprecated \STautoround*{2} with autoround=2
• proper decimal formatting to display exactly 2 decimal places
• add xfp package to provide the \fpeval{} function for floating-point calculations
• siunitx package to provide advanced number formatting used in the \STprintnum definition (autoround=2 only rounds calculations to 2 decimal places, but doesn't display trailing zeros)