added generic plugins: from-pyfunc, pyfunc-filter, to-pyfunc

docs/pyfunc.md

@@ -0,0 +1,49 @@
+No library can dream of offering all the required functionality. Especially for one-off tasks, it makes no sense to 
+develop a whole new plugin library. Hence, there are the following generic plugins that allow the user to utilize 
+custom Python functions:
+
+* reader: `from-pyfunc` - takes a single string as input and outputs an iterable of image containers (as per specified data type)
+* filter: `pyfunc-filter` - takes a single image container or an iterable of them as input and outputs a single container or an iterable of them (as per specified input and output data types)
+* writer: `to-pyfunc` - processes a single image container or an iterable of them as per specified data type and an optional split name
+
+In order to use such a custom function, they must be specified in the following format (option: `-f/--function`):
+
+```
+module_name:function_name
+```
+
+If the code below were available through module `my.code`, then the function specifications would be as follows:
+
+* reader: `my.code:pyfunc_reader`
+* filter: `my.code:pyfunc_filter`
+* writer: `my.code:pyfunc_writer`
+
+
+```python
+from typing import Iterable
+from idc.api import ImageClassificationData, make_list, flatten_list
+
+# reader: generates image classification containers from the path   
+def pyfunc_reader(path: str) -> Iterable[ImageClassificationData]:
+    return [ImageClassificationData(source=path)]
+
+# filter: simply adds a note to the meta-data
+def pyfunc_filter(data):
+    result = []
+    for item in make_list(data):
+        if not item.has_metadata():
+            meta = dict()
+        else:
+            meta = item.get_metadata()
+        meta["note"] = "filtered by a python function!"
+        item.set_metadata(meta)
+        result.append(item)
+    return flatten_list(result)
+
+# writer: simply outputs name and meta-data and, if present, also the split
+def pyfunc_writer(data: ImageClassificationData, split: str = None):
+    if split is None:
+        print("name: ", data.image_name, ", meta:", data.get_metadata())
+    else:
+        print("split:", split, ", name:", data.image_name, ", meta:", data.get_metadata())
+```

mkdocs.yml

@@ -20,6 +20,7 @@ nav:
         - Image segmentation: image_segmentation.md
         - Filter usage: filters.md
         - Docker usage: docker.md
+        - External functions: pyfunc.md
     - Additional libraries:
         - Image augmentation: imgaug.md
         - Image statistics: imgstats.md