add minimum version number for numba support

[mmcky]

This PR adds 0.36.2 as a minimum numba version required for installation in setup.py

numba is one of the key dependencies for QuantEcon.py. Given our user base -- it may be helpful to alternatively write a version check in the base __init__.py file which would give us the opportunity to issue a more useful error message based on conda.

Could check if numba.__version__ > minimum value otherwise print out instructions to upgrade such as conda install numba=0.37. Any comments welcome.

[coveralls]

Coverage remained the same at 95.165% when pulling d856534 on add-numba-version-msg into 35f41b1 on master.

[oyamad]

I think adding a minimum version in setup.py (i.e., the current status of the PR) is sufficient (isn't it always the case that the user who updates QE to the version that contains this patch will automatically update Numba to the minimum version, if below it?).

Another issue: should we list all the requirements, including numpy and scipy, in install_requires in setup.py?

[mmcky]

isn't it always the case that the user who updates QE to the version that contains this patch will automatically update Numba to the minimum version, if below it?

thanks @oyamad. You're right - that should be true although I haven't tested it.

I guess I was thinking of only minimum dependencies that require version constraints, such as numba where the project uses many of the latest features available. But you're probably right - we should have the full list of dependencies (just without version number if not required).

[oyamad]

Another related issue: What is the status of pip-requirements.txt? (And what is the role of that file?)

For example, statsmodels is no longer used in the library.

[mmcky]

Merging now will address other comments in #410