Fix boreholes.field_from_file function to work with 1 borehole

[j-c-cook]

The boreholes.field_from_file(filename) function does not work when there is only one borehole defined in the file.

[MassimoCimmino]

The cause may simply be that numpy.loadtxt(fname) returns a 1d array whe nthere is a single line in the file:

pygfunction/pygfunction/boreholes.py

Lines 848 to 849 in 88a7643

	# Load data from file
	data = np.loadtxt(filename)

A possible fix is to use the ndmin argument to ensure the returned array is two-dimensional:

    # Load data from file
    data = np.loadtxt(filename, ndmin=2)

[MassimoCimmino]

The following snippet produces an error:

>>> import pygfunction as gt
>>> file = np.savetxt('test.txt', np.array([[ 0., 0., 150., 4., 0.075]]), delimiter=' ', header='H\tD\tr_b\tx\ty')
>>> gt.boreholes.field_from_file('test.txt')
IndexError: invalid index to scalar variable.

The proposed change solves the issue.