Drop Readout.value decimal type in favor of float

DESIGN.md

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+DESIGN
+======
+
+Below is a list of design decisions. The justification is to be consulted
+whenever a change is considered, to avoid regressions.
+
+### Data type for DB storage of numeric values (`decimal` vs `float`)
+
+* among database engines supported (by Rails), SQLite offers storage of
+  `decimal` data type with the lowest precision, equal to the precision of
+  `REAL` type (double precision float value, IEEE 754), but in a floating point
+  format,
+    * decimal types in other database engines offer greater precision, but store
+      data in a fixed point format,
+* biology-related values differ by several orders of magnitude; storing them in
+  fixed point format would only make sense if required precision would be
+  greater than that offered by floating point format,
+    * even then, fixed point would mean either bigger memory requirements or
+      worse precision for numbers close to scale limit,
+        * for a fixed point format to use the same 8 bytes of storage as IEEE
+          754, precision would need to be limited to 18 digits (4 bytes/9 digits)
+          and scale approximately half of that - 9,
+    * double precision floating point guarantees 15 digits of precision, which
+      is more than enough for all expected use cases,
+        * single precision floating point only guarntees 6 digits of precision,
+          which is estimated to be too low for some use cases (e.g. storing
+          latitude/longitude with a resolution grater than 100m)
+* double precision floating point (IEEE 754) is a standard that ensures
+  compatibility with all database engines,
+    * the same data format is used internally by Ruby as a `Float`; it
+      guarantees no conversions between storage and computation,
+    * as a standard with hardware implementations ensures both: computing
+      efficiency and hardware/3rd party library compatibility as opposed to Ruby
+      custom `BigDecimal` type