Validate netcdf data on missing numbers on import

[kdberends]

Follow-up of #89

In #89 a fix was implemented in CrossSection to replace -999 values (missing number) by NaN. However, the better way is to do this on on import of data from a D-HYDRO netcdf file, by using the missing number value reported by the netcdf file.

Suggested actions:

• Remove the implementation of FM2PROF generate cross-sections with bed level of -999 when missing data is present in 2D netCDF #89
• Modify the existing netCDF in the test file to contain missing numbers in the water level field
• Assert that constructed cross-sections now contain a bed level of -999
• implement a validation on import of D-Hydro data in the Import module
• assert that cross-section do no longer contain bed levels of -999