Update MEIParser to include intervals and contours between neumes & syllables

[dchiller]

OMR Search enables searching of sequences of intervals and contours both within neumes and between neumes. The version of MEIParser introduced in #806 calculated only intra-neume intervals and contours. This PR adds the calculation of inter-neume intervals and contours to the parser.

This PR also adds some typing to the test suite to conform with --strict checking by mypy and some additional subtests of MEIParser.

[jacobdgm]

On the whole, this looks good. I left a couple of suggestions that I think would lead to improved clarity, which are optional, and one question verifying that the code will behave as expected.