The intent is that an Estimator can retrieve the basis terms in an observables array from the `terms` attribute and validate them itself, rather than hard code the allowed terms here. If we do still want to hardcode this in the classes we could add it back, but the current version of expecting subclasses to override ALLOWED_BASIS doesn't work with the current implementation of class methods which done return the subclass type from methods like reshape etc

This changes the mapping key type to be `(tuple[int], str)` instead of str, to allow for sparse representations in the future. Eg instead of `"XIIIY" you can have `((0, 5,), "XY")`. Also removes validation of allowed characters in the strings. Still need to update unit tests if we go with this.