Include Python accessible config descriptions

[raphaelshirley]

Feature request
I would like to be able to see config definitions through Python. This might be achieved through a Python side set of definitions. This could also exist alongside config requirements that automatically check a config for consistency and that requirements are met.

In pseudo code:

Class lephareConfigParameter:
    def __init__(self,name=None,description=None,value=None,assertion=True):
        self.name=name
        self.definition=definition
        self.value=value
        self.assertion=assertion
    def check_value(self,config):
        assert assertion(config,self.value)

def check_err_scale(config,value):
    "Check errscale is the same length as other filter related parameters and that they exist"
    if "FILTER_LIST" not in config:
        print("FILTER_LIST is not present")
        return False    
    elif "FILTER_CALIB" not in config:
        print("FILTER_CALIB is not present")
        return False
    if  len(str(value).split(',')) != len(str(config["FILTER_LIST"]).split(','))
        print("FILTER_LIST and ERR_SCALE are different lengths")
        return False
    elif len(str(value).split(',')) != len(str(config["FILTER_CALIB"]).split(','))
        print("FILTER_CALIB and ERR_SCALE are different lengths")
        return False
    return True

allowed_config=dict(
    # All the allowed configs
    "ERR_SCALE": lephareConfigParameter(
        name="ERR_SCALE",
        description="The error scaling. Must be equal in length to FILTER_LIST and FILTER_CALIB",
        assertion=check_err_scale,
    )
         
]

required_config=[
    #configs that are required
    "ERR_SCALE",
]

def check_config(config):
    for c in config:
        assert c in allowed_config
        allowed_config[c].check_value()
    for c in required_config:
        assert c in config

Before submitting
Please check the following:

• I have described the purpose of the suggested change, specifying what I need the enhancement to accomplish, i.e. what problem it solves.
• I have included any relevant links, screenshots, environment information, and data relevant to implementing the requested feature, as well as pseudocode for how I want to access the new functionality.
• If I have ideas for how the new feature could be implemented, I have provided explanations and/or pseudocode and/or task lists for the steps.

[raphaelshirley]

RAIL params example:

https://github.com/LSSTDESC/rail_base/blob/main/src/rail/core/common_params.py

[raphaelshirley]

... which is based on CECI parameters:

https://github.com/LSSTDESC/ceci/blob/master/ceci/config.py