Input file linting/validation

[wpbonelli]

Suggested by @pya in MODFLOW-ORG/modflow6#2591 (comment). Excellent idea IMO, cf #81. Not sure whether devtools or flopy is the proper place. Could support functions like

• formatting input files
• linting input files (e.g. deprecated or redundant options)
• checking if an input file is compatible with a given MF6 version
• checking which MF6 version(s) an input file is compatible with