Red Panel

• Better documentation.
• In the function "EstPSTR", now the initial value for "gamma" is allowed. The user can freely choose either "gamma" or "delta" to input for the estimation.
• Estimation of a linear panel regression model with fixed effects.
• The initial values can now be chosen automatically and will be returned after estimation.
• New function "plot_transition" which use the ggplot2 package to plot the estimated transition function.
• The package ggplot2 imported.
• A new data set "sunspot" included.
• Some bugs fixed.