Major context rework and dryml.{ml_framework} addition

* Total rework of context management
* Add new dryml.tf, dryml.torch, and dryml.jax modules which can add
  special methods for those frameworks

Author: krafczyk

src/dryml/context/__init__.py

@@ -1,5 +1,6 @@
 from .context_tracker import active_context, use_context, \
-    set_context, clear_context, context_check, \
+    add_context, set_context, clear_context, \
+    context_check, \
     ContextError, InsufficientResourcesError, \
     ContextAlreadyActiveError, NoContextError, \
     WrongContextError, ContextIncompatibilityError, \
@@ -13,6 +14,7 @@
     active_context,
     use_context,
     set_context,
+    add_context,
     clear_context,
     context_check,
     ResourceSpec,