Adding hpcg experiment class

[august-knox]

Description

Adding hpcg experiment class. Contains strong & weak scaling, & caliper variant.

Dependencies: Currently using david's branch https://github.com/daboehme/hpcg.git

Fixes issue(s): None

Type of Change

• {x} Adding a system, benchmark, or experiment
• { } Modifying an existing system, benchmark, or experiment
• { } Documentation update
• { } Build/CI update
• { } Benchpark core functionality

Checklist:

If adding/modifying a system:

• { } Create a new directory for the system and a new system.py file
• { } Add a new dry run unit test in .github/workflows
• { } System appears in System Specifications table in docs catalogue section

If adding/modifying a benchpark:

• {x} Add a new application.py and (maybe) package.py under a new directory
  for this benchmark
• {x} Configure an experiment
• {x} Benchmark appears in Benchmarks and Experiments table in docs catalogue
  section

If adding/modifying a experiment:

• {x} Extend experiment.py under existing directory for specific benchmark
• {x} Define a single node and multi-node experiments

If adding/modifying core functionality:

• { } Update docs
• { } Update .github/workflows and .gitlab/ci unit tests (if needed)

[rfhaque]

@august-knox Instead of hard-coding +openmp, we should add the openmp variant to this experiment.