[Feature]: Clarify naming of capacity reserve margin price output file

[JesseJenkins]

Feature description

Rename the ReserveMargin.csv output file to ReserveMarginPrice.csv for clarity.

Motivation

Hourly shadow prices for the capacity reserve margin constraints are currently written to a file called ReserveMargin.csv

The naming of this file is likely confusing, as it has no reference to price and could be interpreted as reporting the margin itself (or total demand for firm capacity per hour).

(We may also want to report the hourly capacity reserve required in firm MW elsewhere in a ReserveMarginRequirement.csv file or some similar name, but that is a different output).

Implementation strategy

Simply rename the file written by output functions.

Additional context

No response