Add DiractDistribution for point "things"

[santiagobadia]

A point load can be mathematically represented as a distribution, e.g., a Dirac delta. It breaks the Gridap flow, since one cannot use Gauss quadratures and numerical integration (what we usually do in FEM) to compute the integral of f*v in that case.

I don't want to consider hacks, e.g., touch the vector entry in a particular node in which you want to put the force (assuming the force is on a node).

Instead, e.g., I would consider a constructor for f = DiracDelta(x_0) that returns a Distribution <: Field that we know cannot be used with integration BUT instead, integrate(f*v,trian,quad) returns v(x).