Add possibility to read and store vertical observations

[otzi5300]

Read observations with fixed location and varying depth. Handles dfs0, nc and dataframe.

store data in 1d, eg:
time = [t0, t0 t0, t1, t1, t1, ..., tn, tn, tn]
dep = [0,5,10, 0, 6, 11, ...., 0, 6, 10] (can vary in time)
data = [1, 3, 4, 1, 2, 3, .... 1, 3, 5]