Add visualizations for processed monthly GAUSS data and regression coefficients (Closes #6)

[leahgibson]

Description:
This PR addresses the issue raised in #6 where there were no built-in visualization tools for the data processing steps. This change adds two Python files for plotting the monthly GAUSS data for a given variable and year and for plotting the range, IQR, and mean of the linear regression model runs that estimate a gridded map of each variable given a global temperature output by FaIR. At this time, both functions only work with the variables --tas and --pr. 

Changes Made:

• Added the file scripts/monthly_gauss.py to visualize the four netCDF files produced by processing the monthly GAUSS data
• Added the file scripts/visualize_map_fit.py to visualize the range, IQR, and mean of the regression coefficients from the 100 emulator runs in the function fit_map.py
• Edited README.md to reflect these visualization features

Related Issue:
Closes #6