Dmvtoasim

Add a somewhat trivial function: generate DM variations in TOASIM called 'add_dm'.

Can be optimized still in several ways:
1) Just add a flag to add_rednoise that makes it into DM variations
2) Or make it a separate function that (like add_rednoise) generates the waveform, and then multiplies that with Dk/v^2
3) We might want to get rid of the (12\pi^2) that is still in this definition