[eigen] a couple of RFCs

[Benoit Jacob <jacob.benoit.1@xxxxxxxxx>]

Hi,

1) we currently have a template machine_epsilon<Scalar>() that is
redundant with std::numeric_limits at least for standard numeric
types.
Should we:
a) keep our own machine_epsilon<Scalar>(), and make its
specializations for standard types be wrappers around
std::numeric_limits?
or
b) remove machine_epsilon<Scalar>() and instead use
std::numeric_limits directly? This would require the user to add
custom specializations of
std::numeric_limits if he wants to support custom numeric types. That
seems OK, right? Also note that in StableNorm.h we're already using
std::numeric_limits.

2) in IO.h, by default we output matrices with a "default format" that
uses 4 digits precision. This has repeatedly annoyed people, as they
would expect
cout.precision(n);
cout << matrix;
to work. I suggest that the default value for "precision" be -1, and
that this special value means "use the current precision of the
stream".
Worse: we apply the precision to the stream and don't modify it back
to the original value, so outputting a matrix to a stream has the very
unexpected effect of altering its precision. OK to change that so we
save and restore the stream precision?

Benoit