Allow LocalDensity objects to hand out adouble-versions of themselves

Currently the user code that plugs together the assembler for a
particular problems has to know that ADOL-C is used internally
to compute certain derivatives.  In shows because certain objects
need to be instantiated with adouble as the number type.  One such
example is all objects derived from LocalDensity.

However, if this is to be avoided, then assemblers need to have
a way to turn a (e.g.) 'double' version of a density into an
'adouble' one.  This commit provides this way, by adding a
'makeActiveDensity' method.