Fix broken diameter-range validation in getLineProps

[gaoflow]

Two bugs prevent getLineProps from validating the nominal diameter against a material's d_min / d_max bounds.

Bug 1 — d referenced before assignment

In getLineProps (helpers.py) the diameter-range checks use d before it is defined:

# Check valid diameter ranges
if mat['d_max'] >= 0 and d > mat['d_max']:   # d not defined yet!
    raise Exception(...)
if mat['d_min'] >= 0 and d < mat['d_min']:   # d not defined yet!
    raise Exception(...)

# calculate the relevant properties for this specific line type
d = dnommm*0.001          # <-- d is only assigned here, AFTER the checks

So for any material whose d_min >= 0 or d_max >= 0, the call raises UnboundLocalError: cannot access local variable 'd' instead of the intended out-of-range Exception. Fixed by moving the d = dnommm*0.001 conversion above the checks.

Bug 2 — d_max typo in loadLineProps

output[mat]['d_max'] = getFromDict(props, 'd_dmax', default=-1.0)  # reads the wrong key

The key 'd_dmax' never exists, so a user-specified d_max is silently dropped to the -1 "disabled" default and the upper-bound check never fires. Fixed by reading 'd_max'.

The two bugs masked each other: because d_max always fell back to -1, the d > mat['d_max'] branch of Bug 1 never executed with the default database, so the crash only surfaced via d_min.

Reproduction (before this PR)

from moorpy.helpers import getLineProps, loadLineProps

# Bug 2: user-set d_max is dropped
src = {"lineProps": {"mat": {"mass_d2": 100.0, "MBL_0": 0.0, "MBL_d": 1e6,
                             "d_min": 0.0, "d_max": 0.2}}}
print(loadLineProps(src)["mat"]["d_max"])     # -> -1.0  (should be 0.2)

# Bug 1: UnboundLocalError instead of the intended range error
src2 = {"lineProps": {"mat": {"mass_d2": 100.0, "MBL_0": 0.0, "MBL_d": 1e6,
                              "d_min": 0.05}}}
getLineProps(10.0, "mat", source=src2)        # -> UnboundLocalError: ... 'd'

After this PR

• d_max from the source is honoured.
• An out-of-range diameter raises the intended Exception ("… less than the min valid value …" / "… greater than the max valid value …") for both bounds.
• In-range diameters and the default property database (no d_min/d_max set) are unaffected.

Verification

• New regression test tests/test_line.py::test_getLineProps_diameter_limits covers the in-range success path and both the d_min and d_max out-of-range Exceptions. It fails on dev (UnboundLocalError) and passes with this fix.
• Full test suite: 29 passed.

[shousner]

These are helpful bug fixes that were not addressed when these pieces of code were first implemented, and, as stated, do not come up during normal MoorPy operations.