add test for lake

[OnnoEbbens]

Added a test for the lake package. The test covers a simple case with 2 lakes, evaporation and one like overflowing in the other.

[bdestombe]

Great worrk Onno!

Similarly to time_conversion, we should check the length unit is meters to deal with the dimension specific constants of Manning's eq. length_conversion should be fixed to 1 as we model in meters.

The manual says the following:

to 3.28081, 1.0, and 100.0 when using length units (LENGTH UNITS) of feet, meters, 
or centime�ters in the simulation, respectively. LENGTH CONVERSION does not 
need to be specified if no lake outlets are specified or LENGTH UNITS are meters.

Other than that, looks good to me!

[rubencalje]

Great worrk Onno!

Similarly to time_conversion, we should check the length unit is meters to deal with the dimension specific constants of Manning's eq. length_conversion should be fixed to 1 as we model in meters.

The manual says the following:

to 3.28081, 1.0, and 100.0 when using length units (LENGTH UNITS) of feet, meters, 
or centime�ters in the simulation, respectively. LENGTH CONVERSION does not 
need to be specified if no lake outlets are specified or LENGTH UNITS are meters.

Other than that, looks good to me!

Good remark. We do not add a length unit to ds right now, so I added a todo-comment, so we can merge this Pull Request now.