Allow overwriting location of site_specific directory

[hiker]

This will allow one app (e.g. lfric_atm) to use different compiler flags.

[hiker]

Experiments with lfric_atm show that it is sufficient to just support importing from two different (relative) directories: first try importing Config from apps_specific/..., if that doesn't work, fall back to site_specific/....
This way there is no change to any suites not requiring application specific settings.

LFRic_atm can then be configured using multiple inheritance. Using site_specific the way they are already defined in lfric_core, the lfric_atm apps can use the following:

apps_specific/target/default/config,py:

from site_specific.default.config import Config as DefaultSiteConfig

class Config(DefaultSiteConfig):
    def __init__(self):
                super().__init__()

apps_specific/target/nci_gadi/config.py:

from site_specific.nci_gadi.config import Config as DefaultSiteConfig
from apps_specific.default.config import Config as DefaultAppsConfig

class Config(DefaultAppsConfig, DefaultSiteConfig):
    def __init__(self) -> None:
        super().__init__()

This then leads to the following execution order:

AppsConfig nci_gadi  -->
AppsConfig default  -->
SiteConfig nci_gadi  -->
SiteConfig default  -->

For example, setting up compiler-specific flags can be done in SiteConfig-default, then modified by SiteConfig-nci, then AppsConfig-default, finally AppsConfig-nci.

[hiker]

Closed in #591.