Added parameterized tests for addition and subtraction of quantities with offset units
Added parameterized tests for multiplication and true division of quantities with offset units
Added tests for inplace-operations
Addition of a new error: OffsetUnitCalculusError

This error is raised on operations with offset units that have an ambiguous
result (instead of DimensionalityError).
Fixed two wrong results in tests for addition of offset units
Dropped creation of delta_dimension and delta_units for units without offset

The dimension delta_temperature was dropped because having two dimensions
for temperature has no benefit but only complicates the implementation of
calculations with quantities that have an offset unit. For temperature,
delta_kelvin and delta_degR do no longer exist. There are now only two
delta_units left, delta_degC and delta_degF, which are also of dimension
"temperature".
Implemented correct addition and subtraction for offset units

All tests for addition and subtraction for quantities with offset units pass.
Other (older) tests partly fail and need to be cleaned up: "delta_kelvin",
"delta_degR" and the dimension "delta_temperature" do no longer exist.
Test clean up &, deactivated testing of multiplication/division for offset units

- Remove delta_kelvin in tests,
- Marked tests for multiplication/division of offset untis as known fail
- fixed test_issue66b which asumed that kelvin is an offset unit (which it is not anymore)
Make function _only_multiplicative_units more universal.
Reverted msg in Dimensionality error to old state.
Change: Unit definition creates OffstUnitConverter only if offset != 0 (and not for offset = 0.)
Improved to-method to work better with mixed units

"mixed units" here means a units with offset and units without offset. Now all
sensible conversions are possible. At most one offset unit per dimension is
allowed. Conversion from an offset unit to the delta_unit is also possible.

The latter breaks the addition/subtraction of quantities. A first workaround
for _addsub has been implemented which still needs further improvements.
Clean-up (by mistake missed in last commit)
Offset-Quantities: allowed subtraction, improved tests of inplace-operations

- Tests for new subtraction rule adjusted (before an error was raised)
- Implemented new subtraction rule (needs more work)
- Inplace-tests changed to use np.array (else the inplace operation is not
  performed!)
- Inplace-operations are not yet implemented; tests were deactivated.
- Fixed wrong names of units in unittests (use of plural in UnitContainer
  leads to invalid units)
Fixed tests for operation iadd and isub for quantities with offset-units

Need to be careful to use fresh (not yet inplace-modified) object in each
assert-statement.
Improved implementation of addition/subtraction for quantities with offset units

Added 3 new functions _get_nonmultiplicative_units(quantity),
_get_delta_units(quantity) and _has_compatible_delta(Quantity, unit)
All test for addition and subtraction pass (both standard & inplace operations).
Merge branch 'develop' into feature/_offset_units
Removed test "test_delta_in_diff" that was mark as expected failure

The test is obsolete because of the other tests for operations with offset
units.
Avoid invalid unit names in convert if src and/or dst are given as string

This fixes also #156.
Added new test from fix of #156