change sensitivities to a and b and clarification

[xkykai]

cf issue https://github.com/CliMA/SeawaterPolynomials.jl/issues/30

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (6f70669) 81.15% compared to head (6f02b90) 81.15%. Report is 3 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #31 +/- ## ======================================= Coverage 81.15% 81.15% ======================================= Files 3 3 Lines 69 69 ======================================= Hits 56 56 Misses 13 13 ``` | [Files](https://app.codecov.io/gh/CliMA/SeawaterPolynomials.jl/pull/31?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CliMA) | Coverage Δ | | |---|---|---| | [src/SeawaterPolynomials.jl](https://app.codecov.io/gh/CliMA/SeawaterPolynomials.jl/pull/31?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CliMA#diff-c3JjL1NlYXdhdGVyUG9seW5vbWlhbHMuamw=) | `40.00% <ø> (ø)` | | | [src/TEOS10.jl](https://app.codecov.io/gh/CliMA/SeawaterPolynomials.jl/pull/31?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CliMA#diff-c3JjL1RFT1MxMC5qbA==) | `90.47% <ø> (ø)` | |

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[glwagner]

There's still issues in TEOS10.jl (I can't suggest changes for those lines)

[glwagner]

I searched for "expansion" in TEOS10.jl and found this docstring has some wrong stuff:

https://github.com/xkykai/SeawaterPolynomials.jl/blob/911ff9843b94b7c946e0a8c593de7a8e9c1c8c96/src/TEOS10.jl#L231

[xkykai]

I think I've identified all instances in TEOS10.jl where there were "expansion" and "contraction", and changed them all to "sensitivity", sorry I missed those previously.

[glwagner]

Oh nice let me look again

[glwagner]

Ok, let's merge when tests pass!

[xkykai]

all good to merge?

[glwagner]

oh yeah