Add support for env variable to locate the default MAJIS data directory

At the moment, the location of the Majis data directory needs to be given explicitly:

MajisObservationsList('path/to/data')

It should be useful to support environment variables to provide a default location if none is provided

>>> os.environ['MAJIS_DATA'] = '/data/majis/absolute/path'

>>> MajisObservationsList().path
'/data/majis/absolute/path'

Note: the input path is currently not kept in MajisObservationsList.