update notation q -> L

[trontrytel]

The original paper uses symbol q to denote the mass of a tracer per volume of air (named mass mixing ratio). In our documentation of the 1-moment and 2-moment schemes we used q to denote the mass of a tracer per mass of air (named specific humidity). To keep the notation consistent between the 1,2-moment schemes and P3, and to highlight the difference between normalizing by air mass or volume, we denote the mass of a tracer per volume of air as L (named content).

[codecov[bot]]

Codecov Report

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

Project coverage is 96.77%. Comparing base (497016e) to head (fe661ef). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #428 +/- ## ======================================= Coverage 96.77% 96.77% ======================================= Files 40 40 Lines 1396 1396 ======================================= Hits 1351 1351 Misses 45 45 ``` | [Components](https://app.codecov.io/gh/CliMA/CloudMicrophysics.jl/pull/428/components?src=pr&el=components&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CliMA) | Coverage Δ | | |---|---|---| | [src](https://app.codecov.io/gh/CliMA/CloudMicrophysics.jl/pull/428/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CliMA) | `98.76% <100.00%> (ø)` | | | [ext](https://app.codecov.io/gh/CliMA/CloudMicrophysics.jl/pull/428/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CliMA) | `69.79% <ø> (ø)` | |