Optimise sigma calculation code in dials.integrate

[jbeilstenedmands]

Replace

selection = flex.size_t(range(i0, i1))
val = x.select(selection)

with functionally identical
val = x[i0:i1]

Gives significant performance improvement as the second case is within a loop with the target function calculation.
Also remove an unnecessary enumerate.

Just testing on beta-lactamase:
master

6.1: Using 90666 / 91208 reflections for sigma calculation
        Calculating E.S.D Beam Divergence.
7.3: Calculating E.S.D Reflecting Range (mosaicity).
51.1:  sigma b: 0.039018 degrees
         sigma m: 0.056053 degrees

branch

   6.0: Using 90666 / 91208 reflections for sigma calculation
        Calculating E.S.D Beam Divergence.
   7.1: Calculating E.S.D Reflecting Range (mosaicity).
  29.5:  sigma b: 0.039018 degrees
         sigma m: 0.056053 degrees

Verified the output is identical using @graeme-winter's new matching tool.

[dagewa]

That's a remarkable improvement for a simple change. Good spot!

[codecov]

Codecov Report

Merging #1399 into master will increase coverage by 2.56%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1399      +/-   ##
==========================================
+ Coverage   64.70%   67.26%   +2.56%     
==========================================
  Files         616      616              
  Lines       69735    91437   +21702     
  Branches     9546    15771    +6225     
==========================================
+ Hits        45120    61504   +16384     
- Misses      22835    27771    +4936     
- Partials     1780     2162     +382