make DS::MCPMT getter naming consistent

* Change from e.g. GetPMT to GetMCPMT. Getter names generally
  use the class name.

* Also fix a bug introduced in ea080d12b28b5f4b8113d7aaf2cb6017134badbd,
  where using `DBL_MAX` instead of a large double caused segfaults.
  The underlying reason for this apparently nonsense result is
  under investigation.