Add support for multiple TODs in describe_mpi_distribution

[ziotom78]

This PR adds support for multiple TODs in describe_mpi_distribution. Apart from the possibility to include other TODs in the computation of the allocated memory, this lets one to skip the field Observation.tod, in those cases where it is not allocated, through the use of the new keyword tod_names.

There is a small breaking change here, because now the field tod_dtype of the class MpiProcessDescr is a list of strings instead of a plain string. (This is required, because once multiple TODs are present, it is possible that they have different data types.)

• Add support for multiple TODs in describe_mpi_distribution
• Add new tests
• Document the changes