Vectorize call to integral of the error function (_erfint)

[MassimoCimmino]

In heat_transfer.finite_line_source, the integrand function includes a sum of integrals of the error function :

pygfunction/pygfunction/heat_transfer.py

Lines 288 to 294 in 66d8b4c

	if reaSource and imgSource:
	# Full (real + image) FLS solution
	f = lambda s: 0.5 / (H2*s**2) * np.exp(-dis**2*s**2) * (
	_erfint((D2 - D1 + H2)*s) - _erfint((D2 - D1)*s)
	+ _erfint((D2 - D1 - H1)*s) - _erfint((D2 - D1 + H2 - H1)*s)
	+ _erfint((D2 + D1 + H2)*s) - _erfint((D2 + D1)*s)
	+ _erfint((D2 + D1 + H1)*s) - _erfint((D2 + D1 + H2 + H1)*s) )

The sum can be vectorized as follows :

    if reaSource and imgSource:
        # Full (real + image) FLS solution
        p = np.array([1, -1, 1, -1, 1, -1, 1, -1])
        q = np.stack([D2 - D1 + H2,
                      D2 - D1,
                      D2 - D1 - H1,
                      D2 - D1 + H2 - H1,
                      D2 + D1 + H2,
                      D2 + D1,
                      D2 + D1 + H1,
                      D2 + D1 + H2 + H1],
                     axis=-1)
        f = lambda s: 0.5 / (H2*s**2) * np.exp(-dis**2*s**2) * np.inner(p, _erfint(q*s))

This should be more efficient and save some calculation especially for smaller bore fields.

[MassimoCimmino]

Same test cases as in #78.

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Case 1 : Rectangular fields up to 12 by 12

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Case 2 : Number of segments for a 5 by 5 bore field

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Case 3 : Field of 100 boreholes with 2 different lengths