Refactoring __init__

[xandrd]

Addressing #946. All poes tests are ok

[xandrd]

Also added lanl

[xandrd]

Swarm is added as well. However, there are few notes

1. Swarm request data from HAPI, so, there are no obvious force_download option. Nevertheless, it is introduced as follows:

"""
    force_download : bool, default=False
        Swarm data is requested via HAPI. This parameter always ignored and reserved for compatibility.
"""

2. load function modifies prefix.

    for this_probe in probe:
        prefix = 'swarm' + this_probe.lower() + '_'
        if instrument == 'mag':
            dataset = 'SW_OPER_MAG' + this_probe.upper() + '_' + datatype.upper() + '_' + level[1:].upper()
            tvars = hapi(trange=tr, 
                         server=server, 
                         dataset=dataset, 
                         parameters=varnames,
                         suffix=suffix,
                         prefix=prefix)

Since prefix is passed as a parameter, new code is (see implementation of prefix2):

    if prefix:
        prefix2 = prefix
    else:
        prefix2 = ''

    for this_probe in probe:
        prefix = prefix2 + 'swarm' + this_probe.lower() + '_'
        if instrument == 'mag':
            dataset = 'SW_OPER_MAG' + this_probe.upper() + '_' + datatype.upper() + '_' + level[1:].upper()
            tvars = hapi(trange=tr, 
                         server=server, 
                         dataset=dataset, 
                         parameters=varnames,
                         suffix=suffix,
                         prefix=prefix)

[jameswilburlewis]

It all looks good!