sw_fitpower improvements

[RichardWaiteSTFC]

Some improvements to sw_fitpowder after a first round of testing with users:

• Ignores very small errors (less than 10*eps or around 2e-15) - this makes fitting with chi^2 better as these errors were constraining the fit too much.
• Add functions to cut 1D sets from an input 2D data
• Improve functions to estimate and fit background
• Add ability to set the error on background points to an arbitrary value (if they are somehow too small and thus constrain chi^2 too much).

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Codecov Report

Attention: Patch coverage is 74.21875% with 33 lines in your changes missing coverage. Please review.

Project coverage is 42.37%. Comparing base (89c63fa) to head (32aedd1).

Files	Patch %	Lines
swfiles/sw_fitpowder.m	74.21%	33 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #191 +/- ## ========================================== + Coverage 42.14% 42.37% +0.22% ========================================== Files 240 240 Lines 16091 16184 +93 ========================================== + Hits 6782 6858 +76 - Misses 9309 9326 +17 ```

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Test Results

    4 files  ± 0    112 suites  ±0   4m 22s :stopwatch: -17s   720 tests + 7    702 :white_check_mark: + 7  18 :zzz: ±0  0 :x: ±0  2 052 runs  +28  2 016 :white_check_mark: +28  36 :zzz: ±0  0 :x: ±0 

Results for commit 32aedd1b. ± Comparison against base commit 89c63fa0.

:recycle: This comment has been updated with latest results.