Fix LISFLOOD Errors when no lakes or reservoirs in the domain.

[nicola-martin]

When running LISFLOOD over the SEED-FD domains there are errors for the domains that do not contain lakes/reservoirs due to missing expected input files. This is only a problem for warm starts.

This change adds a check to checkers.py so that if there are no lakes or reservoirs present then the input file check will not run for lakes/reservoirs.

The lisflood code already handles domains without lakes or reservoirs within the hydrological_modules/lakes.py and hydrological_modules/reservoir.py scripts so this deals with switching off the options in the model settings. This extra check just deals with the errors that happen before this stage when running checks on input files for model restarts.

The same logic that is used in hydrological_modules/lakes.py and hydrological_modules/reservoir.py, for determining where a lake/reservoir is present, is used in this new change to checkers.py.

Tests carried out:

• lisflood-code/tests run with pytest. All 73 tests passed (fast and slow tests).
• tested in longflow suite. Ran lisflood for the Juba-Shebelle basin and it successfully ran the model including warmstart/restart.

[nicola-martin]

Adding @corentincarton as a reviewer since this change relates to: ec-jrc#163