Use "from" and "to" instead of "lower" and "upper"
Tell the user which arguments are unknown

When an argument is unknonw, SummationReplacer and IntegralReplacer
now print its name.
\unit argument is now just named "u"

Also improved error message for \unit
Added derivative macro
Summation macro inserts variable name correctly.

Previously it would only insert the "wrt" variable name if the
summation also had a lower bound