Refactor code to make it more readable

[demeringo]

Problem

The naming of some functions is not clear, some methods or structs could benefit from renaming.

Solution

• Things mentioning default_impacts (like print_default_impacts_as_json) could be simplified as we now have a single way to get impacts. No need to mention default anymore.
• standard_scan -> estimate (like in cli ?)
• use the same naming between sls routes and metric server ?

CLI options:

the --as-metric option should be a sub-option of the estimate command (not a top level option), basically it it here to precise the format of the estimate to return. something like format: (json|openmetrics) could be better.

hours-use-time parameter could be renamed into use-duration-hours

Not sure about filter-tags... but we should convey the idea that only the resources with tags mentioned will be included

Alternatives

Additional context or elements