WIP: Add mo_os to lib, include path_isfile to mo_read_spatial_data and mo_read_timeseries

Closed Nicola Nadine Döring requested to merge doeringn/mhm:mo_os_inclusion into develop

path_isfile is added to check whether the file that shall be open and read exists. You get a meaningful error message if the file does not exists or the given path does not describe a file and which path it actually is.

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