Resolve "strange bug in quantity.Symbolic -> valueContinuous after concatenation"

Closes #41 (closed)

+fractional/derivative.m

+function [h, t] = derivative(y, p, t0, t, optArg)
+% DERIVATIVE calculates a fractional derivative
+%	[h, t] = derivative(y, p, t0, t, optArg) comput the p-th fractional derivative of a symbolic
+%	expression y. The lower terminal is t0 and the symbolic independent variable is t. By the
+%	name-value-pair argument, the "type" of the fractional derivative can be specified. Use "RL" for
+%	the Riemann-Liouville derivative and "C" for the Caputo derivative.
+arguments
+	y sym
+	p (1,1) double {mustBePositive};
+	t0 (1,1) double;
+	t (1,1) sym;
+	optArg.type = "RL"
+end
+
+if p < 0
+	error('a must be positive! Use fintegral instead?');
+end
+
+if misc.nearInt(p)
+	h = diff(y, round(p));
+	return
+end
+
+n = ceil(p);
+tau = sym( string(t) + "_");
+
+if optArg.type == "C"	
+	h = 1/gamma(n-p) * int((t-tau)^(n-p-1) * subs(diff(y, t, n), t, tau), tau, t0, t);
+	%DaY = 1/gamma(1+n-a) * ((t - t0)^(n-a) * subs( diff(y, n), t, t0) ...
+	%	+ int((t-tau)^(n-a) * subs(diff(y, 1+n), t, tau), tau, t0, t)); % Integration by parts
+	
+elseif optArg.type == "RL"
+	h = 1 / gamma(n-p) * diff( int( (t-tau)^(n-p-1) * subs(y, t, tau), tau, t0, t), t, n);
+else
+	error("The derivative of the type " + optArg.type + " is not implemented. Only C for Caputo and RL for Riemann-Liouville are available")
+end
+
+
+end
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