v0.1.6: Add new functionality to help testing of packages (#22)

- Implemented macro @test_resource(name), which returns full path to the
  test resource (=file), according to the rule that resources of test
  are under `test` directory and subdirectory, which matches to the name
  of the test file.
- Implemented functions `read_mtx_from_file` and `read_mtx_from_string`,
  which can be used to read matrix files exported by ABAQUS, when using
  commands
  ```ABAQUS
  *MATRIX GENERATE, STIFFNESS, MASS, LOAD
  *MATRIX OUTPUT, STIFFNESS, MASS, LOAD
  ```