Add tests for SAM matrix

[duncanhobbs]

This PR adds tests for the SAM matrix mentioned in issue #7.

I added functions to execute.py to check that the matrix is square, that the rows and columns sum to the expected values, and that the row sums and column sums are equal.

test of execute.py

(cge_env) C:\Users\duncan.hobbs\Github\PSL\CGE\open_cge> python execute.py

Distance at iteration  1  is  4.121147867408581e-12
Model solved, Q =  AGR     116.0
OIL     149.0
IND    1432.0
SER    1803.0
dtype: float64

I also added tests of these functions to test_execute.py

test of test_execute.py

(cge_env) C:\Users\duncan.hobbs\Github\PSL\CGE\open_cge\tests> pytest test_execu
te.py
C:\Users\duncan.hobbs\AppData\Local\Continuum\anaconda3\envs\cge_env\lib\site-pa
ckages\pep8.py:110: FutureWarning: Possible nested set at position 1
  EXTRANEOUS_WHITESPACE_REGEX = re.compile(r'[[({] | []}),;:]')
============================= test session starts =============================
platform win32 -- Python 3.7.3, pytest-4.4.0, py-1.8.0, pluggy-0.9.0
rootdir: C:\Users\duncan.hobbs\Github\PSL\CGE
plugins: xdist-1.26.1, pep8-1.0.6, forked-1.0.2
collected 5 items

test_execute.py .....                                                    [100%]

============================== warnings summary ===============================
open_cge/tests/test_execute.py::test_runner
open_cge/tests/test_execute.py::test_runner
open_cge/tests/test_execute.py::test_runner
  C:\Users\duncan.hobbs\AppData\Local\Continuum\anaconda3\envs\cge_env\lib\site-
packages\numpy\lib\type_check.py:546: DeprecationWarning: np.asscalar(a) is depr
ecated since NumPy v1.16, use a.item() instead
    'a.item() instead', DeprecationWarning, stacklevel=1)

-- Docs: https://docs.pytest.org/en/latest/warnings.html
==================== 5 passed, 3 warnings in 1.72 seconds =====================

[jdebacker]

Thanks @duncanhobbs !