Fix reading Excel file that has cells with formulas that return string

[jmrsnt]

This PR is to fix the issue #483

[Jolanrensen]

Thanks for the fix! Might I just request a recursive call instead of listing all cellType->valueTypes twice? Might also make it easier to extend in the future if we get more types. Something like:

private fun Cell?.cellValue(sheetName: String): Any? {
    if (this == null) return null
    fun getValueFromType(type: CellType?): Any? = when (type) {
        CellType._NONE -> error("Cell $address of sheet $sheetName has a CellType that should only be used internally. This is a bug, please report https://github.com/Kotlin/dataframe/issues")
        CellType.NUMERIC -> {
            val number = numericCellValue
            when {
                DateUtil.isCellDateFormatted(this) -> DateUtil.getLocalDateTime(number).toKotlinLocalDateTime()
                else -> number
            }
        }

        CellType.STRING -> stringCellValue
        CellType.FORMULA -> getValueFromType(cachedFormulaResultType)
        CellType.BLANK -> stringCellValue
        CellType.BOOLEAN -> booleanCellValue
        CellType.ERROR -> errorCellValue
        null -> null
    }

    return getValueFromType(cellType)
}

(and renaming "greather" to "greater" in the test :) )

[jmrsnt]

Yes, it makes sense. Thank you for observing, I am sending the requested changes.