use the user-provided number of Gauss points for line elements

[norihiro-w]

Currently the number of Gauss points for lines is fixed to 2, regardless the number given in *.num files. Modifying the code to setting the number from *.num files is possible and easy. However please note that we have to modify a lot of benchmarks.

This is because the modification means we change the default number from 2 to 3 for line elements, as the default number for all element types is 3. Since many benchmarks do not specify the number in *.num files and OGS uses the default number, the change will result in new solutions which are bit different from reference files.