Added the list-element command

model.py --list-elements shows a list of all elements which can be used
in a threat model with pytm.

Why?
I often find my self looking up the exact names of the elements and
there doc string.

Currently the output looks like this.

    Actor          -- An entity usually initiating actions
    Asset          -- An asset with outgoing or incoming dataflows
    Boundary       -- Trust boundary groups elements and data with the same trust level.
    Dataflow       -- A data flow from a source to a sink
    Datastore      -- An entity storing data
    ExternalEntity --
    Lambda         -- A lambda function running in a Function-as-a-Service (FaaS) environment
    Process        -- An entity processing data
    Server         -- An entity processing data
    SetOfProcesses --

pytm/pytm.py

@@ -492,6 +492,21 @@ def _describe_classes(classes):
         print()
 
 
+def _list_elements():
+    """List all elements which can be used in a threat model with the corisponding description"""
+    def all_subclasses(cls):
+        """Get all sub classes of a class"""
+        subclasses = set(cls.__subclasses__())
+        return subclasses.union(
+            (s for c in subclasses for s in all_subclasses(c)))
+
+    elements = list(sorted(all_subclasses(Element), key=lambda c: c.__name__))
+    max_len = max((len(e.__name__) for e in elements))
+    for sc in elements:
+        doc = sc.__doc__ if sc.__doc__ is not None else ''
+        print(f'{sc.__name__:<{max_len}} -- {doc}')
+
+
 def _get_elements_and_boundaries(flows):
     """filter out elements and boundaries not used in this TM"""
     elements = set()
@@ -983,6 +998,9 @@ def process(self):
         if result.describe is not None:
             _describe_classes(result.describe.split())
 
+        if result.list_elements is not None:
+            _list_elements()
+
         if result.list is True:
             [print("{} - {}".format(t.id, t.description)) for t in TM._threats]
 
@@ -1864,6 +1882,9 @@ def get_args():
     _parser.add_argument(
         "--describe", help="describe the properties available for a given element"
     )
+    _parser.add_argument(
+        "--list-elements", action="store_true", help="list all elements which can be part of a threat model"
+    )
     _parser.add_argument("--json", help="output a JSON file")
     _parser.add_argument(
         "--levels",