load in mixed input formats (json, iod, uk, etc...)

[hornig]

currently, we will only be able to have one type of format to be load in. it will be either only iod or only json, or only csv. an example is here https://github.com/aerospaceresearch/orbitdeterminator/blob/master/orbitdeterminator/kep_determination/positional_observation_reporting.py#L69

what we will need at one point is to be able to mix several input files for multi station orbit determination.