Create ICT: automatically calculate dataIntervalCode?

For the user's convenience, I think the dataIntervalCode can be calculated automatically when IVAR is set. For example like

# let xarr be IVAR as np.array, dtype=float;

# calculate dx as unique diffs in X,
# unique with floats might fail, so add a round to n decimals (e.g. 4):
dx = np.unique(np.diff(xarr).round(4))
# dx is 0 if there's more than one unique diff
dx = dx[0] if dx.size == 1 else 0

OPTIONAL; see issue #14 (closed):

# use an integer if dx is close to its integer value, else float:
dx = int(dx) if np.isclose(dx, int(dx)) else dx

• To make this a reliable feature, a reasonable number of decimals to round to must be determined
• Add test

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The other way around, this feature could be used to verify the dataIntervalCode in an imported ICARTT file.