Reduce cyclomatic complexity functions

[briankfitzwater]

spreadsheet/spreadsheet.go:166:1: cyclomatic complexity 15 of func `(*Sheet).setCell` is high (> 10) (gocyclo)
func (s *Sheet) setCell(cell *xlsx.Cell, val interface{}) {
^
inv/inv.go:294:1: cyclomatic complexity 30 of func `typeToSheet` is high (> 10) (gocyclo)
func typeToSheet(items interface{}) (string, error) {
^
inv/inv.go:242:1: cyclomatic complexity 12 of func `(*Inv).aggregate` is high (> 10) (gocyclo)
func (inv *Inv) aggregate() error {
^