Lisflood: Refactor _create_lisflood_config to accept other formats than .nc for MaskMap

[SarahAlidoost]

Currently, in lisflood module, in _create_lisflood_config, it is assumed that MaskMap has .nc format. But MAskMap can have other formats like .map. This line needs a refactoring.

[sverhoeven]

Yep, if we want to use https://github.com/ec-jrc/lisflood-usecases as example parameter set we need support for mask.map , see eWaterCycle/ewatercycle-lisflood#12

[SarahAlidoost]

As mentioned in eWaterCycle/ewatercycle-lisflood#12, we cannot use other use-cases for now. However, I think the lisflood class can be refactored to avoid .rstrip('.nc').