PCV5: rename input to `img` in `pcv.spectral_index.egi`

**Is your feature request related to a problem? Please describe.**
In #1593 we updated EGI to allow for both RGB and HSI input, but renaming the parameter to the more general `img` would be a breaking change, so wait to update this until v5.0. 

**Additional context**
Parameter naming consistency increases usability and we try to make parameter names informative to the type of input expected. 