Switch ThirdParty PETSc configuration to use pkg-config

Issue/feature addressed

On Fedora PETSc could not be found. This is because it uses a different directory layout than FindPETSc.cmake accommodates.

Proposed solution

The PETSc developers prefer us all to use pkg-config through CMake, so switch to this.

Implementation

We now first find pkg-config, then use the PKG_SEARCH_MODULES macro to find PETSc.

Tests

Notes

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)