Extension to left and interval censored data

[bentaylor1]

The package as it stands can only cope with right censored data. To get left/interval censored data functionality requires:

• an extension to the maximum likelihood fitting methods currently implemented in the package flexsurv, on which spatsurv depends to get initial parameter estimates
• left/interval censored code in the likelihood/gradient functions will need to be debugged and checked
• post processing code will need to be checked