Eigen NumTraits specialisation doesn't define max_digits10()

The NumTraits specialisation which Boost multiprecision provides as part of its Eigen integration is missing a few member functions. One of these is max_digits10 which is required by Eigen 5 for printing matrices containing multiprecision scalars.

Corresponding Eigen issue:

https://gitlab.com/libeigen/eigen/-/issues/2985

where it seems the solution is to either inherit from the base class (to bring in a default) or explicit implement the relevant methods.

Locate the Eigen integration's NumTraits specialisation and compare it with the base class and the corresponding Eigen issue 2985. Check how Eigen 5 requests max_digits10 when printing matrices containing multiprecision scalars, then validate that the missing member functions are available and matrix printing works.

Written by the indexing model from the issue text.