Why is scenic and dynamic San Miguel de Allende called “the Heart of Mexico”? The simple answer would be its place on the map — smack dab in the middle of the country, in the state of Guanajuato.   But the reality is that Mexico’s metaphorical heart beats in this charming colonial city for many reasons:  The country’s crucible of freedom, the cradle of its independence movement, lies in the urban core.…