check elevation of highest pressure level in initialization #56

In the case the user misses pressure level high enough in the climate forcing dataset, the problem is identified only at the downscaling step. We can add a small check in the topoclass object initialization checking we have sufficient pressure levels.

• check elevation of the highest pressure levl against highest DEM elevation (Zplevel > Zdem)
• implement an ERROR statement suggesting which pressure level to get.

for task 1: check the $min(Z_{highest plevel} (x,y,time)) > max(Z_{dem}(x,y))$