Convert sampleCount <-> floating or -> long long explicitly ...

... A non-narrowing conversion out to long long is a necessity, but the
conversions to float and double are simply conveniences.

Conversion from floating is explicit, to avoid unintended consequences with
arithmetic operators, when later sampleCount ceases to be an alias for an
integral type.

Some conversions are not made explicit, where I expect to change the type of
the variable later to have mere size_t width.

include/audacity/Types.h

@@ -69,6 +69,11 @@ public:
 
    operator type () const { return value; }
 
+   float as_float() const { return value; }
+   double as_double() const { return value; }
+
+   long long as_long_long() const { return value; }
+
    sampleCount &operator += (sampleCount b) { value += b.value; return *this; }
    sampleCount &operator -= (sampleCount b) { value -= b.value; return *this; }
    sampleCount &operator *= (sampleCount b) { value *= b.value; return *this; }