Defaults to read-only mode for loaded coverages

'r' mode is the default for loaded coverages.  'a' remains the default for newly created coverages.  To write to loaded coverages, the mode flag must be specified and be non read-only ('a', 'w', or 'r+')

Updates documentation for SimplexCoverage constructor

Addresses [OOIION-668](https://jira.oceanobservatories.org/tasks/browse/OOIION-668)