calc_FadingCorr() doesn't check length of inputs

[mcol]

The age.faded and g_value parameters are expected to be of length 2, but this is not enforced. This leads to warnings and NAs in the results:

calc_FadingCorr(age.faded = 0.1, g_value = 5.0,
                tc = 2592000,  tc.g_value = 172800, n.MC = 10, seed = 1)

 >> Fading correction according to Huntley & Lamothe (2001)
 >> g-value re-calculated for the given tc

 .. used g-value:	5.312 ± NA %/decade
 .. used tc:		8.214e-05 ka
 .. used kappa:		0.0231 ± NA
 ----------------------------------------------
 seed: 			1
 n.MC: 			10
 observations: 		0
 ----------------------------------------------
 Age (faded):		0.1 ka ± NA ka
 Age (corr.):		0.1169 ka ± NA ka
 ----------------------------------------------

 [RLum.Results-class]
	 originator: calc_FadingCorr()
	 data: 2
 	 .. $age.corr : data.frame
	 .. $age.corr.MC : numeric
	 additional info elements:  1
Warning messages:
1: In rnorm(n.MC.i, mean = g_value[1], sd = g_value[2]) : NAs produced
2: In rnorm(n.MC.i, mean = age.faded[1], sd = age.faded[2]) : NAs produced

[mcol]

If tc or tc.g_value is given a negative input, the following unhelpful message is given:

calc_FadingCorr(age.faded = c(1, 0), g_value = c(5, 1), tc = -2592000)

# [calc_FadingCorr()] Error: No solution found, NULL returned: this usually happens for very large,
#   unrealistic g-values, please consider another model for the fading correction