added the LSMR solver as an option

[jacobwilliams]

Fixes #16

[codecov]

Codecov Report

Merging #17 (c06e797) into master (3367ca7) will increase coverage by 0.06%.
The diff coverage is 44.44%.

❗ Current head c06e797 differs from pull request most recent head 6d34f47. Consider uploading reports for the commit 6d34f47 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
+ Coverage   75.28%   75.34%   +0.06%     
==========================================
  Files           1        1              
  Lines         356      365       +9     
==========================================
+ Hits          268      275       +7     
- Misses         88       90       +2