add features for observation.value_as_number

[ginberg]

see #325

[AniekMarkus]

Thanks - this works!

[anthonysena]

Looks good @ginberg! Upon review, I had a small suggestion which is to rename inst/sql/sql_server/MeasurementValue.sql to inst/sql/sql_server/DomainValueAsNumber.sql since you've made this query more generic to go beyond the Measurement domain. I wouldn't worry about checking the inner workings of the query (i.e. some of the temp tables use #meas_). Let me know what you think otherwise this is ready to merge from my side.

[ginberg]

@anthonysena thanks for reviewing! good idea to make the name more generic, I will make an update