Add 'value type' property to formulas

[Nicklas Nordborg]

The specification for spot data export says that the value type needs to be specified for data columns. Eg.

[sdata]
ch1 float
ch2 float
weight  float
flag  int

This is fine for intensities, extra values, raw data, etc. but not for formulas since we don't know which value type a formula generates. We need to add a property to the Formula item that specifies the value type. Note that there are four sub-types of formulas and it is only the "COLUMN_EXPRESSION" type that has an unknown value type. The other three types should produce known types (INTENSITY_EXPRESSION=float, INTENSITY_TRANSFORMATION=float, COLUMN_RESTRICTION=boolean).

For updating existing formulas, the simplest thing is to allow null values and have the server admin set the proper value type. Or... we could try to parse/pattern match the formulas to try to guess the value type... I don't know which is better... I guess it depends on how good the automatic detection is.

[Nicklas Nordborg]

(In [5208]) Fixes #1450: Add 'value type' property to formulas