Suggestion: Add Error Formulas for SWAT+ Performance Metrics

[debpal]

It would be useful to add the following error formulas for computing performance metrics of SWAT+ simulations against observed data:

error_dict = {
    'NSE': 'Nash-Sutcliffe Efficiency',
    'KGE': 'Kling-Gupta Efficiency', 
    'MSE': 'Mean Squared Error',
    'RMSE': 'Root Mean Squared Error',
    'PBIAS': 'Percent Bias',
    'MARE': 'Mean Absolute Relative Error'
}

I have prepared some code for this feature but will wait to integrate it until pull request #30 is merged.

[zepholus]

Resolved in PR #59