Add SRMs using Rsrat

[okamumu]

This PR is to add models by importing from Rsrat (https://github.com/okamumu/Rsrat). Modifications are in the following.

• Add model files for only time data
  • Frechet: SRM whose failure time distribution is an extreme-value distribution (Frechet type).
  • Gamma: SRM whose failure time distribution is a gamma distribution.
  • GO2: Goel-Okumoto with EM
  • Gompertz: SRM whose failure time distribution is a Gompertz distribution (the mean value function becomes a Gompertz curve).
  • GompertzMin: SRM whose failure time distribution is a Gompertz minimum distribution
  • ISS: Inflection S-shaped model
  • LLogis: Log-logistic model
  • LNorm: Log-normal model whose failure time distribution is a log-normal distribution.
  • Pareto: SRM whose failure time distribution is a Pareto (type II) distribution.
  • TNorm: Truncated-normal model whose failure time distribution is a truncated normal distribution.
  • Wei2: Weibull model with EM.
• Add scripts to install and load Rsrat in the files: server.R and install_script.R. The devtools is required to install Rsrat.