Remove optional numba installation code

[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.

[coveralls]

Coverage increased (+0.2%) to 85.967% when pulling 2d3aa9a on remove-optional-numba into b0cb89a on master.

[oyamad]

@mmcky Yes, please remove the try-except block, and use_jit is fine with me.

[mmcky]

@oyamad Thanks - I removed the try-except block and updated xrange to range in the functions.

[coveralls]

Coverage increased (+0.1%) to 85.954% when pulling f32e468 on remove-optional-numba into b0cb89a on master.

[coveralls]

Coverage increased (+0.8%) to 86.667% when pulling d4861b2 on remove-optional-numba into b0cb89a on master.

[mmcky]

Looking at the diff this re-base doesn't look right. I will be closing this PR and implementing the simpler set of changes that remove the code that supported optional numba.