Make public symbols explicit (grimme-lab#114)

Closes grimme-lab#112

Author: awvwgk

src/mctc/data.f90

@@ -46,6 +46,7 @@ module mctc_data
    use mctc_data_paulingen, only : get_pauling_en
    use mctc_data_vdwrad, only : get_vdw_rad
    implicit none
+   private
 
    public :: get_atomic_rad, get_covalent_rad, get_pauling_en, get_vdw_rad
 end module mctc_data

src/mctc/env.f90

@@ -44,6 +44,11 @@ module mctc_env
       & is_unix, is_windows
    use mctc_env_timer, only : timer_type, format_time
    implicit none
-   public
+   private
+
+   public :: sp, dp, wp, i1, i2, i4, i8
+   public :: error_type, fatal_error, mctc_stat
+   public :: get_argument, get_variable, is_unix, is_windows
+   public :: timer_type, format_time
 
 end module mctc_env

src/mctc/env/accuracy.f90

@@ -15,7 +15,9 @@
 !> Numerical storage size parameters for real and integer values
 module mctc_env_accuracy
    implicit none
-   public
+   private
+
+   public :: sp, dp, wp, i1, i2, i4, i8
 
    !> Single precision real numbers
    integer, parameter :: sp = selected_real_kind(6)