Add .clang-format

After a bunch of testing, this Clang Format configuration is quite close
to what is done _most of the time_ in this repository. The only thing
I've found which it breaks consistently is when an UDT is instanciated
inline, the braces are split:
```
struct bar
{} foo;
```
becomes
```
struct bar
{
} foo;
```
and (maybe worse cause that's how it's often written in this project)
```
struct bar
{}
// comment
foo;
```
becomes
```
struct bar
{
}
// comment
foo;
```
. This along with disallowing the rest of multiple operator seperated
items in argument position to flow to the last line if they are
neccesary to split giving:
```
throw invalid_argument("Adiar requires at least "
                       + std::to_string(minimum_memory / 1024 / 1024) + " MiB of memory");
```
instead of
```
throw invalid_argument("Adiar requires at least "
                       + std::to_string(minimum_memory / 1024 / 1024)
		       + " MiB of memory");
```

Except for that, all changes I've observed it making is within
"normal" for the codebase.

.clang-format

@@ -0,0 +1,55 @@
+BasedOnStyle: LLVM
+Language: Cpp
+
+AlignAfterOpenBracket: Align
+AlignConsecutiveAssignments:
+    Enabled: true
+    AcrossEmptyLines: false
+    AcrossComments: false
+    AlignCompound: true
+    PadOperators: true
+AlignOperands: AlignAfterOperator
+AllowAllArgumentsOnNextLine: false
+AllowAllParametersOfDeclarationOnNextLine: false
+AllowShortBlocksOnASingleLine: Always
+AllowShortCaseLabelsOnASingleLine: true
+AllowShortFunctionsOnASingleLine: All
+AllowShortIfStatementsOnASingleLine: AllIfsAndElse
+AllowShortLambdasOnASingleLine: All
+AllowShortLoopsOnASingleLine: true
+AlwaysBreakAfterReturnType: All
+AlwaysBreakTemplateDeclarations: Yes
+BinPackArguments: false
+BinPackParameters: false
+BraceWrapping:
+    AfterCaseLabel: false
+    AfterClass: true
+    AfterControlStatement: Never
+    AfterEnum: true
+    AfterFunction: true
+    AfterNamespace: true
+    AfterStruct: true
+    AfterUnion: true
+    AfterExternBlock: true
+    BeforeCatch: false
+    BeforeElse: false
+    BeforeLambdaBody: false
+    BeforeWhile: false
+    IndentBraces: false
+    SplitEmptyFunction: false
+    SplitEmptyRecord: false
+    SplitEmptyNamespace: true
+BreakBeforeBinaryOperators: NonAssignment
+BreakBeforeBraces: Custom
+BreakBeforeTernaryOperators: true
+ColumnLimit: 100
+ConstructorInitializerAllOnOneLineOrOnePerLine: true
+Cpp11BracedListStyle: true
+FixNamespaceComments: false
+IndentWidth: 2
+MaxEmptyLinesToKeep: 1
+NamespaceIndentation: All
+PointerAlignment: Left
+SpaceAfterTemplateKeyword: false
+# type above function name
+# short if on same line