Significant improvements!

• Simplified error analysis to safe approach of analyzing the diffusion constants over runs.
• Improved fitting of the curves to focus on the central linear portion. There are reasonable defaults but the user can adjust as needed.
• Provided a combined average and error bars when both the MSF approach and Helfand moments are used.
• Capture temperature, pressure, and cell size from the MD step, providing 1/L as a result since the true diffusion constants are found by extrapolating to 1/L = 0.
• Provided control over the number of steps for the expensive numerical integration in the Helfand moments, providing a reasonable default of 1000.