lib: split real FC/field into single and double prec FC/field

lib: split real FC/field into single and double prec FC/field

This commit removes the BT_FIELD_CLASS_TYPE_REAL field class and the
corresponding fields and adds BT_FIELD_CLASS_TYPE_SINGLE_PRECISION_REAL
and BT_FIELD_CLASS_TYPE_DOUBLE_PRECISION_REAL field classes and their
fields.

This seems like cleaner way to express the different types of floating
point representations and while keeping the door open for other
(smaller and/or larger) representations.

The test_field.py testcases for `_SinglePrecisionRealField` are
implemented using a default value of 52.0 which can be converted from
Python float(double precision) to C float(single precision) without loss
of precision. It's the best way I found to support the
`_TestNumericField` test cases without scattering specific conditions
for `_SinglePrecisionRealField`

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I6057e1626b71fee5dce1c374bda3faf3451b5fef
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2099
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>