The default constructor of the FieldVector has intentionally avoided calling a
constructor for the contained data to eliminate the potential overhead of that
initialization.
During implementation of a new creation mode of the BCRSMatrix, it was discovered
that this behavior is problematic due to its inconsistency: If the data type
contained in the FieldVector is a non-trivial type, a default-constructed
FieldVector will be fully initialized. On the other hand, with a primitive data
type it will be in an uninitialized state, potentially causing undefined behavior.

As this behavior might be very unintuitive for users and made implementing the new,
more efficient creation method for the BCRS matrix very difficult to implement, this
patch changes the FieldVector default constructor to always default-construct its
contents regardless of its type. While this change does introduce a possible
performance regression (for primitive types, the compiler now has to zero out the
memory), the overhead has been measured to be minimal.

The actual implementation prefers to use the new C++11 unified initialization
syntax and falls back to std::fill for older compilers. The distinction was made
because newer compilers generate much more efficient code when using unified
initialization, and for user-defined types, this approach avoids first calling the
default constructors and then the copy assignment operator.