Improve UserFunction_Class

[shishiousan]

In the current implementation, the Get method for UserFunction_Class parses a lua function for every call.

This causes an increase in execution time when the Get method is called many times.

The following solutions may be possible.

• implementing the basic functions (polynomial and well-known expression, etc.) by Fortran and calling it.
• suppressing the re-parsing of lua by keeping parsed status (skip call lua_close()).