Update NEWS.md

Update NEWS.md

BigFloat compatibility for univariate optimization, fixes #271 (#336)

* BigFloat compatibility for univariate optimization, fixes #271

The univariate optimization code unintentionally assumed that the choice
variable was a Float64.  Specifically, the interfaces accommodated an
AbstractFloat, but then would crash when reporting the results because of
missing typecasts.

This patch introduces approriate typecasts and test cases.

* fix typecasts -- needed for abs_tol, rel_tol (not lower and upper)

* refactor univariate tests

* drop all Float64 fields from UnivariateOptimizationResults

Update NEWS.md for v0.7.3. (#325)