Added documentation for most modules

Only Spinors and Dirac are missing.

docs/src/flow.md

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+# Gradient flow
+
+The gradient flow equations can be integrated in two different ways:
+1. Using a fixed step-size integrator. In this approach one fixes the
+   step size $\epsilon$ and the links are evolved from
+   $V_\mu(t)$ to $V_\mu(t +\epsilon)$ using some integration
+   scheme. 
+1. Using an adaptive step-size integrator. In this approach one fixes
+   the tolerance $h$ and the links are evolved for a time $t_{\rm
+   end}$ (i.e. from $V_\mu(t)$ to $V_\mu(t +t_{\rm end})$)
+   with the condition that the maximum error while advancing is not
+   larger than $h$.
+   
+In general adaptive step size integrators are much more efficient, but
+one loses the possibility to measure flow quantities at the
+intermediate times $\epsilon, 2\epsilon, 3\epsilon,...$. Adaptive
+step size integrators are ideal for finite size scaling studies, while
+a mix of both integrators is the most efficient approach in scale
+setting applications. 
+
+## Integration schemes
+
+```@docs
+FlowIntr
+wfl_euler
+zfl_euler
+wfl_rk2
+zfl_rk2
+wfl_rk3
+zfl_rk3
+```
+
+## Integrating the flow equations
+
+```@docs
+flw
+flw_adapt
+```
+
+## Observables
+
+```@docs
+Eoft_plaq
+Eoft_clover
+Qtop
+```
+