Add LbExec implementation

This merge request is adds the lbexec command proposed in #198 (closed) and the associated Options class. After Moore and DaVinci are adapted I'll make a follow-up merge request to remove the ApplicationOptions class and start cleaning up/adapating the tests.

This turned into quite a lot of code when compared to the prototype to however the majority of it is test code and error handling. In particular it tries to give people a more helpful message when the application is already doomed to fail (expand below for examples).

lbexec --help

usage: lbexec [-h] [--dry-run] [--export EXPORT] function options [extra_args ...]

positional arguments:
  function         Function to call with the options that will return the configuration. Given in the form 'my_module:function_name'.
  options          YAML data to populate the Application.Options object with. Multiple files can merged using 'file1.yaml+file2.yaml'.
  extra_args

optional arguments:
  -h, --help       show this help message and exit
  --dry-run        Do not run the application, just generate the configuration.
  --export EXPORT  Write a file containing the full options (use "-" for stdout)

Error handling examples

Wrong specification of the function to run

Click to view

Bad contents of the options.yaml file(s)

Click to view

Tracebacks don't include lbexec's implementation details

Click to view

Related to DPA tasks lhcb-dpa/analysis-productions/LbAnalysisProductions#69 (closed) and lhcb-dpa/analysis-productions/LbAnalysisProductions#68 (closed).