[reSIProcate] Data: disambiguate old preallocate and doube constructors

[david Butcher]

<code>
Data pre(100, true);
</code>
will no longer compile.

Instead use:
<code>
Data pre(100, Data::Preallocate);
</code>

<code>
double d = 0.567;
Data dd(d, 3);
</code>
will no longer compile.

Instead use:
<code>
Data d(d, Data::ThreeDigitPrecision);
</code>

david