StreamDevice: add checksum for M905 pressure controller

The ISO 1745 protocol used by the M905 requires a checksum as follows:

One control byte BCC calculated in the following manner:
Perform an exclusive-OR with all bytes between the STX (not included) and the ETX (included).
- If the obtained byte (in ASCII format) is higher than 32, it can be taken as the BCC.
- If the obtained byte (in ASCII format) is lower than 32, the BCC byte will be obtained by adding 32.

(not: it doesn't say what happens if it is exactly 32)

This is an `%` type checksum that is a little like the `%` checksum in that it optionally adds 32 https://paulscherrerinstitute.github.io/StreamDevice/formats.html#chksum