Improves u'h'r'i'z to UBVRCIC transformation equations for main-sequence stars

We report improved transformation equations between the u'g'r'i'z' and UBVRCIC photometric systems. Although the details of the transformations depend on luminosity class, we find a typical rms scatter on the order of 0.001 mag if the sample is limited to main-sequence stars. Furthermore, we find that an accurate transformation requires complex, multicolor dependencies for the bluer bandpasses. Results for giant stars will be reported in a subsequent paper.