New data and subscript functions

[julienmalard]

Added basic subscript functions (! operator) and external (Excel) data loading functions.

[alexprey]

Hi @julienmalard!
Nice to see new work for pysd library and thanks to you for work!
Can you please merge latest version of master branch to your base branch to make it easy check and approve this pull request.

[julienmalard]

Hello,
Is it better now? Sorry for my inexperience with git (despite years of use...)

[coveralls]

Coverage decreased (-10.4%) to 80.34% when pulling 0cd40b3 on julienmalard:new_functions into bf1b1d0 on JamesPHoughton:master.

[alexprey]

Thanks for contribution!
It can be merged if code coverage are keep on the same level, 10% reducing is really bad situation. I can try to add some tests when have a time, but it really better if you can test your code with new test!

[alexprey]

Hi, @julienmalard
I'm just looking at travis build logs and see that more tests are failed because can't import some modules. You also can check it here: https://travis-ci.org/JamesPHoughton/pysd/jobs/596664050

[alexprey]

@julienmalard and some tests failing for python 3: https://travis-ci.org/JamesPHoughton/pysd/jobs/596664051

[DVRodri8]

@julienmalard are you still working on this? my intention is try to fix this branch, we can work together.

[julienmalard]

@DVRodri8 thank you! I would be quite interested in working together to fix this branch. In particular, I am not quite sure how to add the tests (since they would have to be added to the git submodule).

[julienmalard]

@alexprey appologies for the delay. I am not quite sure what is causing the test errors (in the from . import py_backend from integration_test_vensim_pathway); it runs fine on my local machine. Could this be a Python 2 issue? Thanks!

[enekomartinmartinez]

The content of this PR is already in #223