* Demonstrate deduction guides for normal_distribution.
* Add missing test case.
* add class template argument deduction guides for distributions templated on real type - issue #754
* Remove no-arg tests in test_dist_deduction_guides.cpp - issue #754
GCC-8 and clang 6-8 were unhappy with the no-arg cases, which use the
default template arg, not the deduction guide, anyway.
* remove unused deduction guide for fisher_f - issue #754
Co-authored-by: jzmaddock <john@johnmaddock.co.uk>
