allow string (constants) in expressions

[brentp]

Strings are supported in echtvar by storing an integer index into the list of unique strings. Currently, strings aren't really supported, but
we could pre-populated constants, e.g. for FILTER so, e.g.:

PASS = 0
FAIL = 1
QC = 2
...

then the `echtvar anno expression could use:

FILTER == PASS

but this could lead to collisions so instead we will use:

FILTER == FILTER__PASS