the class takes a boolean parameter as its first template parameter
and a type name as the second. if the boolean parameter is false,
nothing is stored, else an object of the type of the second template
parameter gets created. this mechanism allows to disable member
attributes based on compile time conditions.
The usage semantics of that class are that of a smart pointer class, i.e.,
the equivalent of
```
Foo foo;
foo.bar()
```
is
```
Opm::ConditionalStorage<true, Foo> foo;
foo->bar();
```
If the condition argument for the ConditionalStorage is false, that
code will still compile but an exception is thrown at runtime.
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