Better keyval reading in Python

[Lestropie]

There's currently a mismatch between the implementation of utils.load_keyval() and the relevant comment (which reflects original intent). It ignores all content after comment symbols. However with #1609, numeric text files now have key-value-based comments.

Currently, the only use of that function is to parse the output of transformcalc decomp in population_template.

• Read key-values as plain text; don't do any splitting of lines.
• Fix construction of multi-line fields as lists of strings.
• Give capability to read key-values from the comments themselves, rather than from everything other than the comments.
• Give matrix module functions ability to populate both matrix data and header key-values in a single pass of the file, similar to what is done by the numerical text data read functions in C++.