Tests that require matplotlib should be skipped if it's not installed

[swryan]

Task Description

When Dymos is installed without any of it's extra dependencies (including [test]), a user should still be able to run the tests without errors.

After doing a conda install of Dymos from conda-forge, which does not have the option to install with optional dependencies, and running testflo, a large number of tests fail due to matplotlib not being installed. There is also a test that requires pycodestyle. These tests should be skipped if the required dependency is not installed.