Add powder fitting tutorial

[RichardWaiteSTFC]

Have got a .html file generated but don't know if there are other steps required to publish it.

To-Do

• [x] need to add test for bug fixed
• [x] Some typos

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 42.65%. Comparing base (1b52407) to head (636a358). Report is 15 commits behind head on master.

Files	Patch %	Lines
swfiles/sw_fitpowder.m	25.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #199 +/- ## ========================================== + Coverage 42.37% 42.65% +0.28% ========================================== Files 240 241 +1 Lines 16184 16253 +69 ========================================== + Hits 6858 6933 +75 + Misses 9326 9320 -6 ```

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Test Results

    4 files  ± 0    116 suites  +4   4m 56s :stopwatch: +20s   733 tests +13    715 :white_check_mark: +13  18 :zzz: ±0  0 :x: ±0  2 104 runs  +52  2 068 :white_check_mark: +52  36 :zzz: ±0  0 :x: ±0 

Results for commit 636a3586. ± Comparison against base commit 1b524075.

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