Stretching and warping is probably wrong for non zero z_bottom

This [line](https://github.com/CliMA/ClimaCore.jl/blob/7ab0acf8f7d784a69af4bcb8df666ec0164ebc17/src/Meshes/interval.jl#L308) has a typo and is wrong when z_bottom is not 0.

Also, z_bottom is not taken into account when reconstructing `\eta` for warping, e.g. [here](https://github.com/CliMA/ClimaCore.jl/blob/7ab0acf8f7d784a69af4bcb8df666ec0164ebc17/src/Hypsography/Hypsography.jl#L146)

Start by reading the referenced line in src/Meshes/interval.jl and the eta reconstruction around line 146 in src/Hypsography/Hypsography.jl. Trace how z_bottom is represented for stretching and warping, then verify behavior for a nonzero z_bottom. Done means both reported issues are corrected and the existing relevant tests pass.

Written by the indexing model from the issue text.