Remove DataHandle::commit()

From discussion with @leggett on !462 (merged), it seems like the DataHandle::commit() functionality is not used, and that using it is not envisioned anymore. I would thus tentatively propose to remove it.

Can you try this commit in your respective experiment frameworks and tell me how well it goes?

.clang-format

@@ -2,8 +2,7 @@ Language:      Cpp
 BasedOnStyle:  LLVM
 AccessModifierOffset: -2
 AlignConsecutiveAssignments: true
-# Waiting for clang-format-3.9
-# AlignConsecutiveDeclarations: true
+AlignConsecutiveDeclarations: true
 AlignEscapedNewlinesLeft: false
 AlignTrailingComments: true
 AllowAllParametersOfDeclarationOnNextLine: true
@@ -43,6 +42,10 @@ SpacesBeforeTrailingComments: 1
 SpacesInAngles:  false
 SpacesInCStyleCastParentheses: false
 SpacesInParentheses: true
+# Waiting for clang-5
+# SplitEmptyFunction: false
+# SplitEmptyRecord: false
+# SplitEmptyNamespace: false
 Standard:        Cpp11
 TabWidth:        8
 UseTab:          Never