Improvements to input methods and initial documentation for methane functions

[knoiva-indecon]

Improvements/fixes to methods for inputs:

• Added step in "utils_importinputs.R" functions to standardize special characters (e.g., "\." or "") to spaces (i.e., " ") in inputs with region names. Also converted dots in region names in input population scenario to spaces.
• Added steps to clear memory: Added gc() statement to free up unused memory between steps
• Fixed issues with formatting input scenarios for SV module:
  • Added helper function to "utils.R" to zero out temperature or SLR scenarios based on a reference year. New helper function adds a step where non-ID and value columns are joined to the zeroed tibble (with a single point with both the ID and value columns set to zero) to correct issue with mismatch between column numbers during row bind stage. This issue was found for SV, since the SV scenarios have a scenario column that is neither an ID column nor a value column.
• Deleted "defaultResults.rda" (and also deleted step in YML creating the default results) since this dataframe is now quite large and the results are easy to recreate with run_fredi()
• Updated YML to create default scenarios for methane
• Initialized methane documentation
• Updated coefficients and method for calculating mortality. New method uses the natural log of population instead of the relative difference between population and RFF population
• Updated default methane scenario to scenario from FAIR (documentation not yet updated)
• Moved a line standardizing region for driver values to an earlier step, since this only needs to be done for ozone inputs

[cahartin]

@knoiva-indecon are you still waiting to merge this in? i tagged @emcduffie as a code reviewer

[knoiva-indecon]

@knoiva-indecon are you still waiting to merge this in? i tagged @emcduffie as a code reviewer

There are some additional edits on this branch so I was adding some additional small edits here before merging.