[mlpack-git] [mlpack/mlpack] static_assert about IsLMetric is too restrictive (#744)

[Li Dong]

I want to set a custom metric to implement periodic boundary condition as

```c++
class PeriodicMetric : public mlpack::metric::LMetric<2, true> {
public:
    PeriodicMetric() {}

    template <typename VecTypeA, typename VecTypeB>
    static double Evaluate(const VecTypeA &x, const VecTypeB &y) {
        arma::vec::fixed<2> d = x - y;
        // Take into account the periodic boundary condition.
        double tmp1, tmp2;
        for (int i = 0; i < 2; i++) {
            tmp1 = fabs(d[i]);
            tmp2 = 1 - tmp1; // Axis span is 1 in this case.
            if (tmp1 > tmp2) {
                d[i] = d[i] > 0 ? -tmp2 : tmp2;
            }
        }
        return sqrt(dot(d, d));
    }
};
```

When I used it with RangeSearch class, the compiler complains:

```
.../mlpack/core/tree/hrectbound.hpp:59:3: error: static assertion failed: HRectBound can only be used with the LMetric<> metric type.
   static_assert(meta::IsLMetric<MetricType>::Value == true,
   ^~~~~~~~~~~~~
```

I have commented that static_assert in `mlpack/core/tree/hrectbound.hpp` and everything is fine.

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