Support additional Number types (like Percentage)

I was experimenting with adding a wrapper around Doubles for percentages, since that's something many other table solutions support.

Something like this works for basic usage:
```kotlin
data class Percentage(val value: Double, val noDecimals: Int = 2) : Number(), Comparable {
override fun toByte(): Byte = value.toInt().toByte()
override fun toDouble(): Double = value
override fun toFloat(): Float = value.toFloat()
override fun toInt(): Int = value.toInt()
override fun toLong(): Long = value.toLong()
override fun toShort(): Short = value.toInt().toShort()
override fun compareTo(other: Percentage): Int = value.compareTo(other.value)
override fun toChar(): Char = value.toInt().toChar()

private fun Double.roundToNoDecimals(noDecimals: Int): Double =
round(this * 10.0.pow(noDecimals)) / 10.0.pow(noDecimals)

override fun toString(): String {
val percentage = value * 100.0
val double =
if (noDecimals <= 0) {
round(percentage).toLong()
} else {
percentage.roundToNoDecimals(noDecimals)
}
return "$double%"
}
}

fun Number.toPercentage(noDecimals: Int = 2): Percentage = Percentage(this.toDouble(), noDecimals)
```

However, since this is a `Number`, I was under the impression we supported converting it (among other things) right out of the gate. Unfortunately I was wrong:


To fully support any new `Number` implementation, we need:
- Decide on a "default" number type to convert unknowns to. I suggest `Double`.
- filled in `Number` in impl/convert.kt::createConverter
- Check statistics for `Number` cases: https://github.com/Kotlin/dataframe/issues/558
- Column Arithmatics for `Number` columns

Start in impl/convert.kt::createConverter and determine how unknown Number implementations should convert, using Double as the proposed default. Review the statistics cases tracked in issue #558, then inspect column arithmetic for Number columns. Done means conversion, statistics, and arithmetic support the new Number types such as Percentage.

Written by the indexing model from the issue text.