Extract explicit pre-search filter function into a separate module

Details
--
- Move explicit_filters function into separate module under search_filter
- Update signature of explicit filter to take and return query, entries, embeddings
- Use this explicit_filter func from search_filters module in query

Reason
--
Abstraction will simplify adding other pre-search filters. E.g datetime filter

src/search_filter/explicit_filter.py

@@ -0,0 +1,46 @@
+# Standard Packages
+import re
+
+# External Packages
+import torch
+
+
+def explicit_filter(raw_query, entries, embeddings):
+    # Separate natural query from explicit required, blocked words filters
+    query = " ".join([word for word in raw_query.split() if not word.startswith("+") and not word.startswith("-")])
+    required_words = set([word[1:].lower() for word in raw_query.split() if word.startswith("+")])
+    blocked_words = set([word[1:].lower() for word in raw_query.split() if word.startswith("-")])
+
+    if len(required_words) == 0 and len(blocked_words) == 0:
+        return query, entries, embeddings
+
+    # convert each entry to a set of words
+    entries_by_word_set = [set(word.lower()
+                             for word
+                             in re.split(
+                                 r',|\.| |\]|\[\(|\)|\{|\}',  # split on fullstop, comma or any brackets
+                                 entry[0])
+                             if word != "")
+                        for entry in entries]
+
+    # track id of entries to exclude
+    entries_to_exclude = set()
+
+    # mark entries that do not contain all required_words for exclusion
+    if len(required_words) > 0:
+        for id, words_in_entry in enumerate(entries_by_word_set):
+            if not required_words.issubset(words_in_entry):
+                entries_to_exclude.add(id)
+
+    # mark entries that contain any blocked_words for exclusion
+    if len(blocked_words) > 0:
+        for id, words_in_entry in enumerate(entries_by_word_set):
+            if words_in_entry.intersection(blocked_words):
+                entries_to_exclude.add(id)
+
+    # delete entries (and their embeddings) marked for exclusion
+    for id in sorted(list(entries_to_exclude), reverse=True):
+        del entries[id]
+        embeddings = torch.cat((embeddings[:id], embeddings[id+1:]))
+
+    return query, entries, embeddings