Clean up pid_to and sysmeta_to functions

[maier-m]

Currently there are:
pid_to_eml_other_entity
pid_to_eml_datatable
pid_to_eml_physical
sysmeta_to_eml_other_entity
sysmeta_to_eml_physical
And the implementation of these don't seem consistent.

I think these could be consolidated to the following with consistent implementation.
sysmeta_to_eml_physical (no changes)
pid_to_eml_physical (no changes)
pid_to_eml_entity

where pid_to_eml_entity could have the following inputs:

• mn
• pid
• entityType = "Other" (if = "dataTable" will output dataTable or else will output otherEntity)
• ... (additional option to add a named objects like "entityName", "entityDescription", etc.)

[dmullen17]

resolved by #91