Blurb::
Specify the maximum percentage (as a decimal) of the eigenvalue energy not captured
by the active subspace representation.

Description::
The \c truncation_tolerance is used in the energy truncation method to identify
an active subspace. The \c truncation tolerance is used in the following equation
to determine the subspace size:
\f[n = \inf \left\lbrace d \in \mathbf{Z} \quad\middle|\quad 1 \le d \le N \quad \wedge\quad 1 - \frac{\sum_{i = 1}^{d} \lambda_i}{\sum_{i = 1}^{N} \lambda_i} \,<\, \epsilon \right\rbrace \f]
where \f$\epsilon\f$ is the \c truncation_tolerance, \f$n\f$ is the estimated subspace size, \f$N\f$ is the size of the full space, and \f$\lambda_i\f$ are the eigenvalues of the derivative matrix.

The default value for \c truncation_tolerance is \f$1\times 10^{-6}\f$.

