## GammaMatrix

A generalised generator of a Clifford algebra.

A generalised generator of a Clifford algebra. With one vector index,
it satisfies
\begin{equation}
\{ \Gamma^m, \Gamma^n \} = 2\,\eta^{mn}\,.
\end{equation}
The objects with more vector indices are defined as
\begin{equation}
\Gamma^{m_1... m_n} = \Gamma^{[m_1}\cdots \Gamma^{m_n]}\,,
\end{equation}
where the anti-symmetrisation includes a division by $n!$.
If you intend to use the `join_gamma`

algorithm, you have to add a
key/value pair `metric`

to set the name of the tensor which acts
as the unit element in the Clifford algebra.