Add Error Message to Required Variables

This MR allows the module-creator to give the user more information about a set of required variables (if they are missing).

Description

Reason for this change is that miniflask allows module-groups to be entangled, in a way that require a module of one group to be loaded in order for another module to be working.
For instance, imagine a module to be a template or base-class for other instanciations that set the base variables according to their specialized features.

This MR gives the possibility to remind the user in such cases that there is such an dependency and the user requires to choose a specialization.

Previously:
The user would get a list of all required parameters, possibly not remembering or knowing about the inter-connection of some modules.

New Behavior:
Additionally, the user gets a message to describe the unmet dependencies.

Things done in this MR

• added an option to mf.register_defaults(..., missing_argument_message=None)

Check all before creating this PR:

• Documentation adapted
• unit tests adapted / created

Example Usage

def register(mf):
    mf.register_default({'somevariable', int}, missing_argument_message="Did you forget to load a module to specialize this base-module?")