drop requirement on pandas in pytest suite

test/noises/residualgas/AplusSuperGas/__init__.py

+from os import path
+
+import numpy as np
+
 from gwinc.ifo import PLOT_STYLE
 from gwinc import noise
 from gwinc import nb
 from gwinc.ifo.noises import Strain, dhdl
-from os import path
-import pandas as pd
 
 
 class Quantum(nb.Budget):
@@ -57,7 +59,8 @@ def ResidualGasScattering_constructor(species_name, tube):
         def load(self):
             bpath = self.load.__code__.co_filename
             fname = path.join(path.split(bpath)[0], 'beamtube_pressure.txt')
-            df = pd.read_csv(fname, sep='\t')
+            dtype = [(name, float) for name in ('H2_Y', 'H2O_Y', 'N2_Y', 'H2_X', 'H2O_X', 'N2_X', 'position_m')]
+            df = np.loadtxt(fname, dtype=dtype)
             pressure_torr = df[species_name + '_' + tube]
             self.tubepos_m = df['position_m']
             self.pressure_Pa = pressure_torr * 133.3

test/noises/residualgas/AplusSuperGas/beamtube_pressure.txt

-H2_Y	H2O_Y	N2_Y	H2_X	H2O_X	N2_X	position_m
+#H2_Y	H2O_Y	N2_Y	H2_X	H2O_X	N2_X	position_m
 3.83730510272139e-09	5.236986290952779e-13	7.94627513146534e-10	3.83730510272139e-09	8.056901986081187e-14	9.608652335698945e-10	0.0
 3.871805744669283e-09	5.1282058746962454e-12	7.864523763900331e-10	3.871805744669283e-09	7.889547499532675e-13	1.033699661246794e-09	19.9725
 3.905186329470217e-09	9.64534866283429e-12	7.782772396335322e-10	3.905186329470217e-09	1.4838997942821969e-12	1.1065340889236938e-09	39.945