Enhancement for Gnuplot_Class

[shishiousan]

Some issues in terms of Gnuplot_Class need to be solved

directory not found can happen when the plt file is created

The class still uses intrinsic write function. It causes this error.
We have to use Txtfile_Class or extend it to solve this issue.

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Execution of gnuplot command shows an error depending on a kind of OS

Some OS has several variants of gnuplot commands which can support a specific
terminal option (For example, wxt is not supported in normal gnuplot command in Fedora. We need to execute gnuplot-wx instead.) .
So we need to implement a utility to set the proper terminal option or gnuplot binary depending on a kind of OS.