edited is_zero for families of distributions in a hack way
fixed bugs in lift for OMS, solve_diff_eqn for families, and _shift_coeffs

for lift there were problems with the fact that accuracy and number of moments don't match for families.
for solve_diff_eqn, similar problem caused need to increase accuracy above given accuracy
for _shift_coeffs, it was failing for 0 power series
changed precision in solve_diff_eqn for families -- only moved from M to M-1 (haven't shifted by additional m)