Add file reader for rrlvl files

docs/references.bib

@@ -414,3 +414,12 @@ @techreport{dere_chianti_2017
   urldate = {2022-08-21},
   langid = {english}
 }
+
+@techreport{young_chianti_2019,
+  title = {{{CHIANTI Technical Report No}}. 24: {{The CHIANTI}} Level-Resolved Recombination Files (Rrlvl)},
+  author = {Young, Peter R.},
+  year = {2019},
+  month = feb,
+  number = {24},
+  urldate = {2024-01-09}
+}

fiasco/io/sources/ion_sources.py

@@ -22,6 +22,7 @@
     'DrparamsParser',
     'DiparamsParser',
     'AutoParser',
+    'RrlvlParser',
 ]
 
 
@@ -461,3 +462,49 @@ def preprocessor(self, table, line, index):
         super().preprocessor(table, line, index)
         # remove the dash in the second-to-last entry
         table[-1][-2] = table[-1][-2].split('-')[0].strip()
+
+
+class RrlvlParser(GenericIonParser):
+    """
+    Level-resolved recombination rates as a function of temperature.
+
+    These files contain the *Direct* radiative recombination rates from
+    the recombining ion to the recombined ion. Note that these files contain
+    the *effective* radiation recombination rate coefficients.
+    A given ion should have either a ``.rrlvl`` file or a ``.reclvl`` file,
+    but not both. For a full description of these files, see :cite:t:`young_chianti_2019`.
+    """
+    filetype = 'rrlvl'
+    dtypes = [int, int, int, int, float, float]
+    units = [None, None, None, None, u.K, (u.cm**3)/u.s]
+    headings = [
+        'Z',
+        'ion',
+        'initial_level',
+        'final_level',
+        'temperature',
+        'rate',
+    ]
+    descriptions = [
+        'atomic number',
+        'ionization state',
+        'level index of the recombining ion',
+        'index of the final level of the transition in the recombined ion',
+        'temperatures at which rates are tabulated',
+        'direct radiative recombination rate coefficients',
+    ]
+
+    def preprocessor(self, table, line, index):
+        # NOTE: Every pair of lines has the same first four entries. On the even
+        # lines, the remaining entries contain the temperatures and on the odd
+        # lines, the remaining entries are the rate coefficients. Thus, every
+        # other line needs to be added to the one above it.
+        line = line.strip().split()
+        if index % 2 == 0:
+            row = line[:4]
+            temperature = np.array(line[4:], dtype=float)
+            row += [temperature]
+            table.append(row)
+        else:
+            rate_coefficient = np.array(line[4:], dtype=float)
+            table[-1].append(rate_coefficient)

fiasco/io/sources/tests/test_sources.py

@@ -23,6 +23,7 @@
     'fe_12.drparams',
     'al_3.diparams',
     pytest.param('fe_23.auto', marks=pytest.mark.requires_dbase_version('>=9')),
+    pytest.param('fe_23.rrlvl', marks=pytest.mark.requires_dbase_version('>=9')),
 ])
 def test_ion_sources(ascii_dbase_root, filename,):
     parser = fiasco.io.Parser(filename, ascii_dbase_root=ascii_dbase_root)