Add NumTraits::digits for Eigen 5 support

[benmwebb]

The random number generator in Eigen 5 calls NumTraits<Scalar>::digits(). This is not currently implemented for CppAD types, causing a compile-time failure. Add such an implementation that simply uses std::numeric_limits<T>::digits. Closes #250.

[bradbell]

Nice job and thanks!

I have a few minor comments to help with future pull requests:

1. Mention the users view of the changes in the whats new for the current year; e.g., see
   4af526f

2. Put the name of the branch you are working on at the beginning of the log messages for the corresponding commits. This makes it easy to see which commits are on the main branch and correspond to a fully functioning CppAD. For example, the log message for the commit above begins with pull_251 because that is the branch I was working on. I use the following utility for this; bin/git_commit.sh and for some other nice features (like caching the previous commit message in git_commit.log).

3. After I merge the pull request, I change the message to reflect that date at which the change became part of the CppAD releases; see
   dcc87a3

[bradbell]

Perhaps we should change the testing of CppAD to use Eigen 5 ?