`boost::posix_time::duration` multiplication/division limited to `int`.

The `operator*()`, `operator*=()`, `operator/()`, and `operator/=()` currently only support `int` parameters [see [`boost::posix_time::duration`](https://github.com/boostorg/date_time/blob/85e637cb325208c2af9af791c3a1948b4888c6cd/include/boost/date_time/time_duration.hpp#L204)].

I tried to use it as follows:

```
std::uint64_t const timedeltaInTensOfNanoseconds = 2793555578;
boost::posix_time::time_duration const duration = boost::posix_time::nanoseconds(10) * timestamp;
// duration.total_nanoseconds() = -15014117180
```
[I did define `BOOST_DATE_TIME_POSIX_TIME_STD_CONFIG`.]

What did work as expected was:

```
std::uint64_t const timedeltaInTensOfNanoseconds = 2793555578;
boost::posix_time::time_duration const duration = boost::posix_time::nanoseconds(10 * timestamp);
// duration.total_nanoseconds() = 27935555780
```

Would it be possible to extend the parameter type for above functions to at least support `unsigned` as well? Or even extend it to 64-bit types? Or make them template-methods [with a concept limiting them to `int`s and `float`s]?