Name of `subset.constant_latitude_interval` (also that of longitude) sounds confusing

[erogluorhan]

In the subset API, we have constant_latitude() and constant_longitude() functions, which work for a single scalar value for either lat or lon as expected. Having the lat/ton interval subsetting functions' names prefixed also with "constant_" sounds a bit confusing, i.e. constant_latitude_interval() and constant_longitude_interval().

I think we should do either of the following ways forward instead:

1. We can rename constant_latitude_interval() and constant_longitude_interval() to latitude_interval() and longitude_interval() (and show deprecation warning for the existing ones that start with"constant_" for a few versions forward). This way is the simplest to implement.
2. Or, maybe we want to combine latitude functions together and longitude ones together as well, i.e. have only one subset.latitude() and let it do both cases of constant latitude and latitude interval depending on how lat arg is fed (a constant value or a tuple). I am not sure if we want to go this route though.