Add missing dirtree texlive package to install.

Issue/feature addressed

Changes to texlive package dependencies resulted in the dirtree TeXlive package not being implicitly installed.

Proposed solution

Add the package explicitly to the list of packages to be installed from the TeXlive distribution.

Implementation

Added dirtree to the package list.

Tests

N/A

Notes

None

Checklist

• Functions and classes, or changes to them, are documented.
• User guide/documentation is updated.
• Changelog is updated.
• Suitable tests added for new functionality.
• Contributed code is correctly formatted. (See the contributing guidelines).
• License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).

Warning

On the 19.07 the code formatting (code style) was standardised using clang-format, over the whole Nektar++ code. This means changes in your branch will conflict with formatting changes on the master branch. To resolve these conflicts , see #295 (closed)