Handle file/compressed/directory structures and file-chunking in FileInput classes

`FileInput` and derived classes like `StringFileInput` can handle lists of files from directory and `glob.glob` parameters. Still all file content is read/passed as a single `Packet`. Also `.zip` files are handled by a dedicated class `ZipFileInput`.

It should be possible to generalize `FileInput` to have derived classes read from files no matter if files came from directory structures, `glob.glob` expanded file lists or .zip files. Even a mixture of these should be handled. For example within NLExtract https://github.com/nlextract/NLExtract/blob/master/bag/src/bagfilereader.py can handle any file structure provided.

A second aspect is `file chunking`: a `FileInput` may split up a single file into Packets containing data structures extracted from that file. For example, `FileInput`s like `XmlElementStreamerFileInput` and `LineStreamerFileInput`
open/parse a file but pass file-content (lines, parsed elements) in
fine-grained chunks on each `read()`. Currently these classes implement this fully
within their `read()` function, but the generic pattern is that they
maintain a "context" for the open/parsed file.

So all in all this issue addresses two general aspects:
- handle any `file-specs`: directories, maps, `Globbing`, zip-files and any mix of these
- handle fine-grained file-chunking: on each invoke()/read() may supply part of a file: a line an XML element etc.

See also issue #49 for additional discussion which lead to this issue.
The Strategy Design Pattern may be applied (many refs on the web).