unit tests: Update to recent API changes.
agilent-dmm: U124x/U125x fixes.
gmc-mh-1x-2x-rs232: Completed energy measurement ranges (V, A, W) for Metrahit 29S.
agilent-dmm: U124xx/U125xx: support 5 more modes.

This adds support for resistance, capacitance, frequency, continuity,
and temperature.
agilent-dmm: Add temperature regex to U125xx parser.
input/wav: Fix broken handling of float32 samples on big endian

Also, make sure that floats are 32 bit even in the case of an
extensible header.
input/wav: Correctly check supported sample size.
scpi/usbtmc: Minor debug output changes.
C++: Remove erroneous stray method.
C++: Fix duplicated shared_ptr creation.
agilent-dmm: U124xx/U125xx: Add current loop sensor support.

Also fixes frequency support.
input/wav: Now really correctly check supported sample size.
C++: Don't spell out arguments to StructureWrapper when not required.
C++: Make StructureWrapper a recurring template, eliminating lots of casts.
C++: Rename StructureWrapper to ParentOwned.
C++: Add UserOwned base template for objects with resources owned by user.
C++: Implement Deleter pattern in UserOwned template.
Add GPIB SCPI backend using linux-gpib and libgpib.
build: Use pkgconfig for libgpib.
scpi/gpib: Rename backend to scpi_libgpib.

libgpib is the userspace component to linux-gpib's kernel modules that
implement low-level interface drivers.

When libsigrok gets userspace GPIB interface drivers, that backend will
be the "official" scpi_gpib.