[Important Fix!] Parse started to removed unselected columns

[Jolanrensen]

Fixed parseImpl() removing unselected columns by reverting implementation to use convert instead of insert.

We didn't have a test that ran parse { someCol }, but I found it running the example in README.md

[koperagen]

Why is it different from original code? isFrameColumn changed

internal fun <T> DataFrame<T>.parseImpl(options: ParserOptions?, columns: ColumnsSelector<T, Any?>) =
    convert(columns).to {
        when {
            it.isFrameColumn() -> it.cast<AnyFrame?>().parse(options)

            it.isColumnGroup() ->
                it.asColumnGroup()
                    .parse(options) { all() }
                    .asColumnGroup(it.name())
                    .asDataColumn()

            it.typeClass == String::class -> it.cast<String?>().tryParse(options)

            else -> it
        }
    }

[Jolanrensen]

@koperagen I changed it last time so all logic of parseImpl lives inside parseImpl, making it easier to debug, understand how it works, and parallelize in the future.