[ENH] Ensure consistent indentation for comments in yaml files, addre…

…ss yamllint warning (#1117)

* ENH: yamllint - ensure consistent indentation for comments

* BF: Address yamllint errors/warnings:

    $> yamllint -c .yamllint.yml src/schema/
    src/schema/rules/tabular_metadata.yaml
      10:23     error    too few spaces before comment  (comments)
      11:11     error    too few spaces before comment  (comments)

    src/schema/rules/checks/func.yaml
      1:1       warning  missing document start "---"  (document-start)

.yamllint.yml

@@ -6,3 +6,5 @@ rules:
   indentation:
     # See https://github.com/yaml/pyyaml/issues/545 for why
     indent-sequences: false
+  comments:
+    level: error

src/schema/rules/checks/func.yaml

@@ -1,3 +1,5 @@
+# Rules for functional data that are not defined in tables.
+---
 PhaseSuffixDeprecated:
   issue:
     code: PHASE_SUFFIX_DEPRECATED

src/schema/rules/tabular_metadata.yaml

@@ -7,8 +7,8 @@ scans:
   - .json
   entities:
     subject: required
-    session: optional # session is required if session is present in the dataset.
-sessions: # This file may only exist if session is present in the dataset.
+    session: optional  # session is required if session is present in the dataset.
+sessions:  # This file may only exist if session is present in the dataset.
   suffixes:
   - sessions
   extensions: