Added few changes
method for linear system of 3 or more eq of 1st order
Methods of linear system of 3eq and 1st order
Added linear system of n equation of 1st order
Added test cases
Made changes in linear system of 3eq 1st order type4
classification of system of 2 nonlinear eq of 1st order
Merge branch 'sysode_nonlinear_order1_2eq' into sysode_nonlinear_order1_2eq