Proposal: `measurementLists` Group as alternative to many `measurementList1`...`measurementList2` indexed groups

[sstucker]

It has been highlighted by #103 that the Indexed Group, described as

Each element of the sub-group is uniquely identified by appending a string-formatted index (starting from 1, with no preceding zeros) in the name, for example, /.../name1 denotes the first sub-group of data element name, and /.../name2 denotes the 2nd element, and so on.

is a wildly inefficient way to structure an HDF5 file.

This draft adds an alternative encoding of the measurementList.

There are a few known issues at this point, such as defining a character to be NaN at the index of channels which lack a particular value.