consolidate similar functions for image and tabular data

[zzsi]

• Loading. https://github.com/kungfuai/kaishi/blob/master/kaishi/tabular.py#L63, https://github.com/kungfuai/kaishi/blob/master/kaishi/util/file.py#L76
• Dedup. https://github.com/kungfuai/kaishi/blob/master/kaishi/util/filters.py#L5, and https://github.com/kungfuai/kaishi/blob/master/kaishi/tabular.py#L122

Perhaps do this by inheritance from an abstract class that covers both image and tabular data.

[mwharton3]

These are both completed (each inherits from the FileGroup class)