Function to check if seq contains only DNA

[natir]

Implement:

• valid_DNA(input: &[u8]) -> bool // If anything in input isn't a A, C, T, G, a, c, t or g return false
• valid_DNA_with_N(input: &[u8]) -> bool // If anything in input isn't a A, C, T, G, N, a, c, t, g, n return false