0.0.5

1. Add "robust" and "one-way clustered" standard error to IVGMM function
2. Add two-step gmm option, "gmm2" option, to the IVGMM function
3. IV2SLS function will be removed beyond version 0.0.5 and will be replaced with IVGMM.

0.0.4

1. update grammar
2. update demean method when the length of category variables list equals 2.
3. add fontsize and figsize option in did model.