Remove code that supported optional numba installation

[mmcky]

This PR removes code that was written to support environments that did't have numba installed. Now that numba is a strict dependency of the package - this code can now be removed. This will simplify how code is written etc.

Updates:

1. markov/gth_solve should we remove the following given support for Python 3.5+:

+#-TODO: Remove as we no longer support Python <3.5?
+try:
+    xrange
+except:  # python3
+    xrange = range

1. gth_solve so as not to change the code to much, I have added a use_jit header to the function signature to allow one to optionally use numpy. @oyamad Is this ok?

Deprecations:

1. The numba_installed package wide variable is no longer available.

[mmcky]

In #246 It was resolved to remove the python2.7 range statements and to keep the use_jit adjustment for gth_solve

[coveralls]

Coverage increased (+0.2%) to 86.667% when pulling f58e625 on remove-optional-numba-code into 4eade8d on master.

[mmcky]

I will leave this open for a day - in case there are any comments on the final diff.

[oyamad]

@jit(nopython=True) will work if the line A = np.asarray(A) is removed (assuming that the input A is an numpy ndarray). (But I suspect we wouldn't get much speedup.)

[mmcky]

Thanks @oyamad Agree. Let's leave this as is.

[mmcky]

Thanks for your comments @oyamad. I will now remove the #TODO statements. In the future we can revisit the @jit statements as numba improves and evolves.

[coveralls]

Coverage increased (+0.2%) to 86.667% when pulling 59bddd5 on remove-optional-numba-code into 4eade8d on master.