Accumulated Dispersion Additions

[cdrios]

Changes:

• Added a function in Link.py to calculate the accumulated dispersion for a given wavelength
• Added 2 variables in Span.py for the dispersion parameter D and dispersion slope S calculated from B_2 and B_3
• Fixed a typo in Link.py comments

Comments:

• B_2 and B_3 currently use the variables dispersion_coefficient and dispersion_slope. This is misleading because these names belong to other quantities in the literature
• The central wavelength used in link.py and span.py needs to be adjusted. I set it to 1550 nm, but it needs to be obtained from the same source as the current B_2 and B_3. I think obtaining D and S directly from a fiber spec sheet is easier than calculating them from B_2 and B_3, but I know that B_2 and B_3 are used in other calculations. I'm not sure that B_2 and B_3 are usually listed on fiber spec sheets. I'll mention this in the next meeting.