[MM] mmc42dj & dj2mmc4 tools

[HYLcool]

There are diverse formats for multimodal datasets. So we provide several format conversion tools, which convert original datasets to the Data-Juicer intermediate format and convert them back. Data-Juicer will accept datasets in Data-Juicer format and process them.

Here we add tools for converting MMC4-like dataset to the target dataset in Data-Juicer format and reverse first. Other formats will be supported in the future.