Support parser interpolation?

[tclune]

Summary of Issue:

The strict format checking currently prevents us from exploiting some of the power of Python configs. In particular it would be useful to have a default section where additional variables could be defined and then used with interpolation in subsequent sections.

I can probably implement as it is mostly loosening some of the protections, but want to gauge the interest before I try.

Expected behavior and actual behavior:

[DEFAULT]
BASE: [email protected]:GEOS-ESM   # ME complains that "BASE" is not a known key.

[GMAO_Shared]
repo_url = ${DEFAULT:BASE}/GMAO_Shared.git
local_path = ./src/Shared/@GMAO_Shared
tag = v1.0.5
sparse = ../../../config/GMAO_Shared.sparse

[GEOSgcm_App]
repo_url = ${DEFAULT:BASE}/GEOSgcm_App.git
local_path = ./src/Applications/@GEOSgcm_App
tag = v1.0.3

should allow us to just edit the BASE line at the top to reflect

Steps to reproduce the problem (should include model description file(s) or link to public repository):

N/A

What is the changeset ID of the code, and the machine you are using:

N/A

have you modified the code? If so, it must be committed and available for testing:

N/A

Screen output or log file showing the error message and context:

N/A