Store Jinja environment in TemplateRenderer

[paulromano]

Description

This PR includes a subtle change to our TemplateRenderer class, which now stores a jinja2.Environment instance. This may be needed down the line for some envisioned features (e.g., special Jinja filters to be used for MCNP) and otherwise doesn't affect functionality.

Two other small changes:

• I added an input_file property to the PluginMCNP class
• There was a small fix needed in plugin_dakota.py that was raising a warning during tests

Checklist:

• My code follows the style guidelines
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)
• I have updated the CHANGELOG.md file (if applicable)
• I have successfully run examples that may be affected by my changes