Miscellaneous changes

Change the following 11 problems:
Utah/AP_Calculus_I/set4_Graphing_and_Maximum-Minimum_Problems/1210s7p15.pg  Add units
UMN/calculusStewartCCC/s_6_4_9.pg  Add solution
UMN/calculusStewartET/s_7_8_32.pg  Add solution
UVA-Stew5e/setUVA-Stew5e-C02S09-DerivAsFunct/2-9-24.pg  Make sure the function is not constant
UVA-Stew5e/setUVA-Stew5e-C01S01-Functions/1-1-51.pg  Clarify: sides *of the base*
UMN/calculusStewartCCC/s_1_1_41.pg  Add link for interval notation
Rochester/setIntegrals4FTC/sc5_4_18a.pg  Fix broken link
Rochester/setVmultivariable6Gradient/ur_vc_6_17.pg  Miscategorized
UVA-Stew5e/setUVA-Stew5e-C02S06-InfLimits/2-6-37g.pg  Fix incorrect answer, and minor input improvements
ASU-topics/setIntegralSubstitution/sc5_5_25.pg  Convert to MathObjects, and make it pickier about absolute values
UCSB/Stewart5_5_3/Stewart5_5_3_54.pg  Convert to MathObjects, and randomize it