[WIP] New caliper modifier implementation

[rfhaque]

Work in progress. This PR changes the caliper modifier implementation (Solves #603)
In the existing caliper modifier, we define caliper modes to collect specific metrics (e.g. cuda, mpi, time etc)
The current benchpark implementation sets these modes at experiment init time. Each of these modes creates an environment variable in the experiment bash script to collect the desired metrics. However, this approach does not facilitate benchmark-specific changes as the caliper modifier is processed during the Ramble setup phase.

In the new impementation, we use the caliper modes specified on experiment init to directly set the required variables in the experiment ramble.yaml. The generated ramble.yaml is functionally equivalent to the one generated by the existing caliper modifier. However, this approach allows for changes to the caliper config at benchmark init time.

• Replace with: A short description of the change, including motivation and context.

Adding/modifying a benchmark (docs: Adding a Benchmark)

• Tags in Ramble's application.py or in repo/benchmark_name/application.py will appear in the docs catalogue
• Add/modify an experiments/benchmark_name/experiment.py to define a single node and multi-node experiments