Remove redundant PB-(S)AM keywords

[sobolevnrm]

• PB-SAM surf = usemesh
• PB-SAM grid2d = write
• PB-SAM dx = write (or read)
• PB-SAM exp could be replaced with generalized version of APBS read
• PB-SAM diff command ordering could be replaced with use of mol id

[sobolevnrm]

@mvaliev can you take this on?