Rename package

[ayushpatnaikgit]

As the general registry doesn't allow 3-letter-word packages, can we rename this to MertonDtD.jl?

@ajaynshah @SusanXKDR

[SusanXKDR]

Yes. For full reference,
Merton, R. C. (1974). On the pricing of corporate debt: the risk structure of interest rates. The Journal of Finance, 29(2), 449–470