Refactoring: conversion functions instead of gnpy.core.units.UNITS

The TL;DR behind this patch is that it's better to have a utility
conversion function instead of having multiplier LUT and open code which
implements the conversion.

The FiberParams handling looked fishy -- apparently, it was keeping the
multiplier around, but it was unconditionally setting the units to
meters, anyway. Given that the units were not being preserved anyway
(everything got converted to meters), and that the multipler was not
used anywhere, let's refactor the code to just convert to meters using
our new utility function, and remove the unused argument.

Change-Id: Id886d409a4046f980eed569265baefd97db841bd

gnpy/core/utils.py

@@ -15,6 +15,7 @@ from csv import writer
 import numpy as np
 from numpy import pi, cos, sqrt, log10
 from scipy import constants
+from gnpy.core.exceptions import ConfigurationError
 
 
 def load_json(filename):
@@ -282,3 +283,29 @@ def automatic_fmax(f_min, spacing, nch):
     196125000000000.0
     """
     return f_min + spacing * nch
+
+
+def convert_length(value, units):
+    """Convert length into basic SI units
+
+    >>> convert_length(1, 'km')
+    1000.0
+    >>> convert_length(2.0, 'km')
+    2000.0
+    >>> convert_length(123, 'm')
+    123.0
+    >>> convert_length(123.0, 'm')
+    123.0
+    >>> convert_length(42.1, 'km')
+    42100.0
+    >>> convert_length(666, 'yards')
+    Traceback (most recent call last):
+        ...
+    gnpy.core.exceptions.ConfigurationError: Cannot convert length in "yards" into meters
+    """
+    if units == 'm':
+        return value * 1e0
+    elif units == 'km':
+        return value * 1e3
+    else:
+        raise ConfigurationError(f'Cannot convert length in "{units}" into meters')