Integrate code for interpolation from chebpol to enable approximation method.

• Add interpolation functions ipol into package to enable approximation method. Also export evaluation functions evaluation to help compare the accuracy between original and approx methods.

Temporarily disable approximation method

• Temporarily disable approximation method due to deprecation of dependent package "chebpol".

Minor Update

1. Add user-defined use.nearPD, so that user can decide if latent correlation matrix should be adjusted to be positive definite automatically.
2. Remove redundant code for checking positive definiteness.
3. Minor correction on types detection to accommodate NA values.

Initial release

1. This is the initial release on Github.
2. This will be second release on CRAN soon.