Computes the \(\ell^p\) norm of an n-dimensional (real/complex)
vector \(\mathbf{x} \in \mathbf{C}^n\)
$$ \left|\left| \mathbf{x} \right|\right|_p = \left( \sum_{i=1}^n
\left| x_i \right|^p \right)^{1/p}, p \in [0, \infty],$$
where \(\left| x_i \right|\) is the absolute value of \(x_i\). For
\(p=2\) this is Euclidean norm; for \(p=1\) it is Manhattan norm. For
\(p=0\) it is defined as the number of non-zero elements in
\(\mathbf{x}\); for \(p = \infty\) it is the maximum of the absolute
values of \(\mathbf{x}\).
The norm of \(\mathbf{x}\) equals \(0\) if and only if \(\mathbf{x} =
\mathbf{0}\).