Beginning to write documentation for build_A.m
Documented proposed change to constraint matrix. Also note a warning about calculation of losses, that might be incorrect
Added section explaining A_lag.
Removed incorrect error about double-counting losses
Added comment about calculating losses after removing loss variables
Merge branch 'matrix_documentation'
Adding .gitignore

Specifies to ignore:
(1) .mat files
(2) directories containing excel files, namely
    (a) all_scenarios*/
    (b) SP\ ?/