Transition to C++17

Release Notes

With this release, the codebase transitions to C++17,
requiring more modern compilers (gcc 7, clang 5) and dependencies.

Minor Changes

• Transition to C++17 (#250)
• Replace header guards w/ '#pragma once' (#253)
• Replace Nose w/ Pytest (#252)
• Replace GoogleTest w/ Catch2 (#251)

Removed Features

• Non-MEF, API-only random deviates (use STL or Boost random directly).

Since v0.16.1

80 commits resulted in 193 files changed, 3321 insertions(+), 4040 deletions(-)

• Core: 96 files changed, 1061 insertions(+), 1613 deletions(-)
• Scripts: 1 file changed, 111 insertions(+), 88 deletions(-)
• GUI: 39 files changed, 240 insertions(+), 443 deletions(-)
• Tests: 40 files changed, 1724 insertions(+), 1727 deletions(-)
• Documentation: 4 files changed, 41 insertions(+), 61 deletions(-)
• Schemas: No change