Added testing for files generated by pygmsh/meshio

This is taken from issue #21.

All credit for these test files and gen.py script goes to @2pt0

assets/pygmsh/ascii.vtk

@@ -0,0 +1,181 @@
+# vtk DataFile Version 5.1
+written by meshio v5.3.0
+ASCII
+DATASET UNSTRUCTURED_GRID
+POINTS 18 double
+0.0 0.0 0.0 1.0 -0.2 0.0 1.1 1.2 0.0 0.1 0.7 0.0 0.3333333333325021 -0.06666666666650042 0.0 0.6666666666657866 -0.1333333333331573 0.0 1.0249999999999424 0.14999999999919245 0.0 1.0499999999998704 0.4999999999981836 0.0 1.074999999999934 0.8499999999990746 0.0 0.766666666667985 1.0333333333339925 0.0 0.433333333334733 0.8666666666673664 0.0 0.050000000000122564 0.3500000000008579 0.0 0.7444729167676052 0.3524793413776178 0.0 0.3781088913238718 0.4816987298113132 0.0 0.7412636346823331 0.6806963451979247 0.0 0.5070791452210437 0.16277273408010906 0.0 0.253704273975508 0.18556095944515594 0.0 0.7797139636550688 0.08823831456107314 0.0
+CELLS 39 94
+OFFSETS vtktypeint64
+0
+2
+4
+6
+8
+10
+12
+14
+16
+18
+20
+22
+24
+27
+30
+33
+36
+39
+42
+45
+48
+51
+54
+57
+60
+63
+66
+69
+72
+75
+78
+81
+84
+87
+90
+91
+92
+93
+94
+CONNECTIVITY vtktypeint64
+0
+4
+4
+5
+5
+1
+1
+6
+6
+7
+7
+8
+8
+2
+2
+9
+9
+10
+10
+3
+3
+11
+11
+0
+10
+13
+14
+13
+12
+14
+10
+3
+13
+8
+2
+9
+4
+5
+15
+9
+10
+14
+3
+11
+13
+6
+7
+12
+8
+9
+14
+7
+8
+14
+12
+7
+14
+15
+5
+17
+5
+1
+17
+1
+6
+17
+12
+13
+15
+13
+11
+16
+15
+13
+16
+11
+0
+16
+0
+4
+16
+6
+12
+17
+12
+15
+17
+4
+15
+16
+0
+1
+2
+3
+CELL_TYPES 38
+3
+3
+3
+3
+3
+3
+3
+3
+3
+3
+3
+3
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+1
+1
+1
+1