Make sure number of initial conditions is correct in desolve_system

[kcrisman]

The following should probably raise an error:

var('t')
A = function('A',t)
B = function('B',t)
de1 = diff(A,t) == -k1*A+k2*B
de2 = diff(B,t) == k1*A-k2*B
sol = desolve_system([de1,de2],[A,B],ics=[500,0])

Or conceivably it should correspond to A(500)=0 and leaving B(500) unevaluated, but in that case it should be checked whether that is really what happens, and well-documented.

CC: @sagetrac-JGuzman

Component: calculus

Issue created by migration from https://trac.sagemath.org/ticket/14877