Add more tests

[palday]

I also simplified the mime_string testing util and made the error catching for eltype conversions better.

[codecov]

Codecov Report

Merging #15 (2a0d9c3) into main (ff4425f) will increase coverage by 33.33%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             main       #15       +/-   ##
============================================
+ Coverage   66.66%   100.00%   +33.33%     
============================================
  Files           1         1               
  Lines          33        37        +4     
============================================
+ Hits           22        37       +15     
+ Misses         11         0       -11     

Impacted Files	Coverage Δ
src/centering.jl	100.00% <100.00%> (+33.33%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ff4425f...2a0d9c3. Read the comment docs.