Add SBE pressure names

[vonopr]

Here is the list of pressure names for .cnv files:

• prM Pressure [db] pr M db User-entry for moored pressure (instrument with no pressure sensor)
• prE Pressure [psi] pr E psi User-entry for moored pressure (instrument with no pressure sensor)
• prDM Pressure, Digiquartz [db] pr M db Digiquartz pressure sensor
• pr50M Pressure, SBE 50 [db] pr50 M db 1st SBE 50 pressure sensor
• pr50M1 Pressure, SBE 50, 2 [db] pr50 M2 db 2nd SBE 50 pressure sensor
• prSM Pressure, Strain Gauge [db] pr M db strain-gauge pressure sensor
• prdM Pressure, Strain Gauge [db] pr M db strain-gauge pressure sensor
• pr Pressure [db] Specific python-ctd name, see Problem while reading old Sea Bird MicroCat SBE 37SM #20

Marked items are recognized by cnv-parser.

[vonopr]

As disscussed in #21, here is a check list of supported/unsopported pessure names. I would like to prepare a Pull Request that adds support of listed names.