Retrievable explanations, Restrict explanations to ConceptMaps (#20)

## What is the goal of this PR?
Explanations can exceed the gRPC message size limits. This change allows retrieving explanations separately from the `ConceptMap` answers themselves, allowing us to break the deep nesting and long messages contained in a single answer. 

The paradigm for retriving an explanation is now as follows:
A `ConceptMap` always contains a `pattern` and a boolean flag `hasExplanation`. If the `ConceptMap` has an explanation, a call can be made using message type `Explanation.Req`, returning a list of `ConceptMap` that correspond to to a single layer of the explanation tree.

Unused `Explanation` messages have been removed from answers other than `ConceptMap`.

## What are the changes implemented in this PR?
* `ConceptMap` contains `conceptMap`, `pattern` and `hasExplanation`
* A new RPC endpoint can be performed by clients using type `Explanation.Req`, returning `Explanation.Res`
* `Explanation` removed from answers other than `ConceptMap`

Author: flyingsilverfin

session/Answer.proto

@@ -38,40 +38,40 @@ message Answer {
 }
 
 message Explanation {
-    string pattern = 1;
-    repeated ConceptMap answers = 2;
+    message Req {
+        ConceptMap explainable = 1;
+    }
+    message Res {
+        repeated ConceptMap explanation = 1;
+    }
 }
 
 message AnswerGroup {
     Concept owner = 1;
     repeated Answer answers = 2;
-    Explanation explanation = 3;
 }
 
 message ConceptMap {
     map<string, Concept> map = 1;
-    Explanation explanation = 2;
+    string pattern = 2;
+    bool hasExplanation = 3;
 }
 
 message ConceptList {
     ConceptIds list = 1;
-    Explanation explanation = 2;
 }
 
 message ConceptSet {
     ConceptIds set = 1;
-    Explanation explanation = 2;
 }
 
 message ConceptSetMeasure {
     ConceptIds set = 1;
     Number measurement = 2;
-    Explanation explanation = 3;
 }
 
 message Value {
     Number number = 1;
-    Explanation explanation = 2;
 }
 
 message Void {

session/Session.proto

@@ -76,6 +76,7 @@ message Transaction {
             PutRole.Req putRole_req = 11;
             PutRule.Req putRule_req = 12;
             ConceptMethod.Req conceptMethod_req = 13;
+            Explanation.Req explanation_req = 14;
         }
     }
     message Res {
@@ -93,6 +94,7 @@ message Transaction {
             PutRole.Res putRole_res = 11;
             PutRule.Res putRule_res = 12;
             ConceptMethod.Res conceptMethod_res = 13;
+            Explanation.Res explanation_res = 14;
         }
     }