Add TEOS10 heat capacity, plus some comments about TEOS10 implementation

[glwagner]

The TEOS10 reference heat capacity is now accessible via

SeawaterPolynomials.TEOS10.teos10_reference_heat_capacity

Here's a description of the TEOS10 reference heat capacity:

from the TEOS10 manual (equation 3.3.3, section 3.3, page 27).

[codecov[bot]]

Codecov Report

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

Comparison is base (0d77c52) 81.15% compared to head (5fe445a) 81.15%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #33 +/- ## ======================================= Coverage 81.15% 81.15% ======================================= Files 3 3 Lines 69 69 ======================================= Hits 56 56 Misses 13 13 ```

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