Improving file handling

[LisaHopcroft]

I've implemented a few things to improve the handling of input .csv files that are not in the format that the R script is expecting. This includes:

• reporting when expected columns are missing
• accommodating different column names
• accommodating different date formats
• reporting that a file is missing

I've also added a few other features that I found useful when developing the features listed above.

• accommodating alternative input/output directories
• validating input parameter draw_figures (should be yes/no)
• printing additional information when verbose=True