make Assemblier be fit for Petrov-Galerkin method
add and pass a penalty based 2d test for mechanical assemblier
add python script related to gitignore
redesign Lagrangle dirichlet BC interface and implements

  * one MFNode has a larangle index instead of indes list
  * add value "1" diag element to main matrix for invalid dirichlet BC dimension

  * test: use python3 script directly instead of json, for mechanical assembliers